<?xml version="1.0" encoding="US-ASCII"?><!--Generated by ThunderDome XBRL - 05:42PM UTC 2016-02-26--><link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple"/>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-11-recent-accounting-pronouncements" xlink:href="bcrx-20151231.xsd#statement-note-11-recent-accounting-pronouncements" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-11-recent-accounting-pronouncements" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NewAccountingPronouncementsAndChangesInAccountingPrinciplesTextBlock" xlink:label="us-gaap_NewAccountingPronouncementsAndChangesInAccountingPrinciplesTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_NewAccountingPronouncementsAndChangesInAccountingPrinciplesTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-10-quarterly-financial-information-unaudited" xlink:href="bcrx-20151231.xsd#statement-note-10-quarterly-financial-information-unaudited" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-10-quarterly-financial-information-unaudited" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_QuarterlyFinancialInformationTextBlock" xlink:label="us-gaap_QuarterlyFinancialInformationTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_QuarterlyFinancialInformationTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-document-and-entity-information" xlink:href="bcrx-20151231.xsd#statement-document-and-entity-information" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-document-and-entity-information" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_AmendmentFlag" xlink:label="dei_AmendmentFlag" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_CurrentFiscalYearEndDate" xlink:label="dei_CurrentFiscalYearEndDate" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentFiscalPeriodFocus" xlink:label="dei_DocumentFiscalPeriodFocus" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentFiscalYearFocus" xlink:label="dei_DocumentFiscalYearFocus" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentPeriodEndDate" xlink:label="dei_DocumentPeriodEndDate" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentType" xlink:label="dei_DocumentType" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityCentralIndexKey" xlink:label="dei_EntityCentralIndexKey" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityCommonStockSharesOutstanding" xlink:label="dei_EntityCommonStockSharesOutstanding" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityCurrentReportingStatus" xlink:label="dei_EntityCurrentReportingStatus" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityFilerCategory" xlink:label="dei_EntityFilerCategory" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityPublicFloat" xlink:label="dei_EntityPublicFloat" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityRegistrantName" xlink:label="dei_EntityRegistrantName" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityVoluntaryFilers" xlink:label="dei_EntityVoluntaryFilers" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityWellKnownSeasonedIssuer" xlink:label="dei_EntityWellKnownSeasonedIssuer" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_TradingSymbol" xlink:label="dei_TradingSymbol" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_EntityRegistrantName" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_EntityCentralIndexKey" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_TradingSymbol" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_CurrentFiscalYearEndDate" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_EntityFilerCategory" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_EntityCurrentReportingStatus" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_EntityVoluntaryFilers" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_EntityWellKnownSeasonedIssuer" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_EntityCommonStockSharesOutstanding" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_EntityPublicFloat" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_DocumentType" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_DocumentPeriodEndDate" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_DocumentFiscalYearFocus" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_DocumentFiscalPeriodFocus" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="dei_AmendmentFlag" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-consolidated-balance-sheets-parentheticals" xlink:href="bcrx-20151231.xsd#statement-consolidated-balance-sheets-parentheticals" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-consolidated-balance-sheets-parentheticals" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="us-gaap_CommonStockSharesAuthorized" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommonStockSharesIssued" xlink:label="us-gaap_CommonStockSharesIssued" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="us-gaap_CommonStockSharesOutstanding" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PreferredStockSharesAuthorized" xlink:label="us-gaap_PreferredStockSharesAuthorized" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PreferredStockSharesOutstanding" xlink:label="us-gaap_PreferredStockSharesOutstanding" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockParOrStatedValuePerShare" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesAuthorized" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PreferredStockSharesOutstanding" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockParOrStatedValuePerShare" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesAuthorized" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesIssued" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockSharesOutstanding" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-consolidated-statements-of-stockholders-equity-parentheticals" xlink:href="bcrx-20151231.xsd#statement-consolidated-statements-of-stockholders-equity-parentheticals" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-consolidated-statements-of-stockholders-equity-parentheticals" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:label="us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementEquityComponentsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_CommonStockMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-1-significant-accounting-policies-and-concentrations-of-risk" xlink:href="bcrx-20151231.xsd#statement-note-1-significant-accounting-policies-and-concentrations-of-risk" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-1-significant-accounting-policies-and-concentrations-of-risk" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AgreementMember" xlink:label="bcrx_AgreementMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CSLMember" xlink:label="bcrx_CSLMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CollateralForCreditMember" xlink:label="bcrx_CollateralForCreditMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ContingentUponEuMarketingApprovalMember" xlink:label="bcrx_ContingentUponEuMarketingApprovalMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CurrencyHedgeAgreementMember" xlink:label="bcrx_CurrencyHedgeAgreementMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_JPRRoyaltySubLLCMember" xlink:label="bcrx_JPRRoyaltySubLLCMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_LaboratoryEquipmentOfficeEquipmentAndSoftwareMember" xlink:label="bcrx_LaboratoryEquipmentOfficeEquipmentAndSoftwareMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_PhaRMANotesMember" xlink:label="bcrx_PhaRMANotesMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RAPIVABMember" xlink:label="bcrx_RAPIVABMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RegulatorySupportRevenueForCanadianAndEUMarketingApprovalsPortionRecognizedRatablyOverExpectedPeriodOfInvolvementMember" xlink:label="bcrx_RegulatorySupportRevenueForCanadianAndEUMarketingApprovalsPortionRecognizedRatablyOverExpectedPeriodOfInvolvementMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RevenueFromSaleOfInventoryToBeRecognizedWhenTheInventoryTransferIsCompleteMember" xlink:label="bcrx_RevenueFromSaleOfInventoryToBeRecognizedWhenTheInventoryTransferIsCompleteMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RoyaltyReceivableMember" xlink:label="bcrx_RoyaltyReceivableMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BuildingMember" xlink:label="us-gaap_BuildingMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CertificatesOfDepositMember" xlink:label="us-gaap_CertificatesOfDepositMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ComputerEquipmentMember" xlink:label="us-gaap_ComputerEquipmentMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="us-gaap_ConcentrationRiskBenchmarkDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:label="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskByTypeAxis" xlink:label="us-gaap_ConcentrationRiskByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="us-gaap_ConcentrationRiskTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="us-gaap_ConcentrationRiskTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CounterpartyNameAxis" xlink:label="us-gaap_CounterpartyNameAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CustomerConcentrationRiskMember" xlink:label="us-gaap_CustomerConcentrationRiskMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeAxis" xlink:label="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="us-gaap_DeferredRevenueArrangementTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForeignExchangeContractMember" xlink:label="us-gaap_ForeignExchangeContractMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="us-gaap_FurnitureAndFixturesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationAxis" xlink:label="us-gaap_HedgingDesignationAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="us-gaap_HedgingDesignationDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="us-gaap_HedgingDesignationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="us-gaap_LongtermDebtTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="us-gaap_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="us-gaap_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NondesignatedMember" xlink:label="us-gaap_NondesignatedMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="us-gaap_ProductOrServiceAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="us-gaap_ProductsAndServicesDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="us-gaap_ProductsAndServicesDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueProductLineMember" xlink:label="us-gaap_SalesRevenueProductLineMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SignificantAccountingPoliciesTextBlock" xlink:label="us-gaap_SignificantAccountingPoliciesTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="us-gaap_TypeOfArrangementAxis" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="bcrx_CurrencyHedgeAgreementMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_EntityDomain" xlink:to="bcrx_JPRRoyaltySubLLCMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:to="us-gaap_CertificatesOfDepositMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:to="bcrx_RoyaltyReceivableMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:to="bcrx_CollateralForCreditMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ComputerEquipmentMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="bcrx_LaboratoryEquipmentOfficeEquipmentAndSoftwareMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_FurnitureAndFixturesMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_BuildingMember" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_TypeOfArrangementAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_AgreementMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember-default" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_CounterpartyNameAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="bcrx_CSLMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_ProductOrServiceAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ProductsAndServicesDomain" xlink:to="bcrx_RAPIVABMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ProductOrServiceAxis" xlink:to="us-gaap_ProductsAndServicesDomain-default" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScenarioUnspecifiedDomain" xlink:to="bcrx_ContingentUponEuMarketingApprovalMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:to="bcrx_RevenueFromSaleOfInventoryToBeRecognizedWhenTheInventoryTransferIsCompleteMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:to="us-gaap_DeferredRevenueArrangementTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:to="bcrx_RegulatorySupportRevenueForCanadianAndEUMarketingApprovalsPortionRecognizedRatablyOverExpectedPeriodOfInvolvementMember" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_LongtermDebtTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="bcrx_PhaRMANotesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="us-gaap_ForeignExchangeContractMember" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_HedgingDesignationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_HedgingDesignationDomain" xlink:to="us-gaap_NondesignatedMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_HedgingDesignationAxis" xlink:to="us-gaap_HedgingDesignationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_ConcentrationRiskByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ConcentrationRiskTypeDomain" xlink:to="us-gaap_CustomerConcentrationRiskMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ConcentrationRiskByTypeAxis" xlink:to="us-gaap_ConcentrationRiskTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:to="us-gaap_SalesRevenueProductLineMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:to="us-gaap_ConcentrationRiskBenchmarkDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_HedgingDesignationAxis" xlink:to="us-gaap_HedgingDesignationDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ProductOrServiceAxis" xlink:to="us-gaap_ProductsAndServicesDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:to="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ConcentrationRiskByTypeAxis" xlink:to="us-gaap_ConcentrationRiskTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:to="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_SignificantAccountingPoliciesTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-2-furniture-and-equipment" xlink:href="bcrx-20151231.xsd#statement-note-2-furniture-and-equipment" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-2-furniture-and-equipment" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentDisclosureTextBlock" xlink:label="us-gaap_PropertyPlantAndEquipmentDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_PropertyPlantAndEquipmentDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-3-royalty-monetization" xlink:href="bcrx-20151231.xsd#statement-note-3-royalty-monetization" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-3-royalty-monetization" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CurrencyDomain" xlink:label="bcrx_CurrencyDomain" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CurrencyDomain" xlink:label="bcrx_CurrencyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CurrencyHedgeAgreementMember" xlink:label="bcrx_CurrencyHedgeAgreementMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_JPRRoyaltySubLLCMember" xlink:label="bcrx_JPRRoyaltySubLLCMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_JapanYenMember" xlink:label="bcrx_JapanYenMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_PhaRMANotesMember" xlink:label="bcrx_PhaRMANotesMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RoyaltyMonetizationMember" xlink:label="bcrx_RoyaltyMonetizationMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RoyaltyMonetizationTextBlock" xlink:label="bcrx_RoyaltyMonetizationTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_Shortfall2013Member" xlink:label="bcrx_Shortfall2013Member" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:label="us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CurrencyAxis" xlink:label="us-gaap_CurrencyAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="us-gaap_FairValueInputsLevel2Member" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForeignExchangeContractMember" xlink:label="us-gaap_ForeignExchangeContractMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationAxis" xlink:label="us-gaap_HedgingDesignationAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="us-gaap_HedgingDesignationDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="us-gaap_HedgingDesignationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:label="us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:label="us-gaap_LegalEntityTypeOfCounterpartyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NondesignatedMember" xlink:label="us-gaap_NondesignatedMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCommitmentsAxis" xlink:label="us-gaap_OtherCommitmentsAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCommitmentsDomain" xlink:label="us-gaap_OtherCommitmentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCommitmentsDomain" xlink:label="us-gaap_OtherCommitmentsDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_DebtInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="bcrx_PhaRMANotesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:to="bcrx_JPRRoyaltySubLLCMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:to="us-gaap_LegalEntityTypeOfCounterpartyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="bcrx_CurrencyHedgeAgreementMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_EntityDomain" xlink:to="bcrx_JPRRoyaltySubLLCMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_OtherCommitmentsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsDomain" xlink:to="bcrx_RoyaltyMonetizationMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_OtherCommitmentsAxis" xlink:to="us-gaap_OtherCommitmentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScenarioUnspecifiedDomain" xlink:to="bcrx_Shortfall2013Member" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel2Member" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_CurrencyAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="bcrx_CurrencyDomain" xlink:to="bcrx_JapanYenMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CurrencyAxis" xlink:to="bcrx_CurrencyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="us-gaap_ForeignExchangeContractMember" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_HedgingDesignationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_HedgingDesignationDomain" xlink:to="us-gaap_NondesignatedMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_HedgingDesignationAxis" xlink:to="us-gaap_HedgingDesignationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_HedgingDesignationAxis" xlink:to="us-gaap_HedgingDesignationDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CurrencyAxis" xlink:to="bcrx_CurrencyDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:to="us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_OtherCommitmentsAxis" xlink:to="us-gaap_OtherCommitmentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="bcrx_RoyaltyMonetizationTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-4-lease-obligations-and-other-contingencies" xlink:href="bcrx-20151231.xsd#statement-note-4-lease-obligations-and-other-contingencies" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-4-lease-obligations-and-other-contingencies" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_BirminghamResearchFacilityMember" xlink:label="bcrx_BirminghamResearchFacilityMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_LongTermLiabilitiesMember" xlink:label="bcrx_LongTermLiabilitiesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="us-gaap_BalanceSheetLocationAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommitmentsAndContingenciesDisclosureTextBlock" xlink:label="us-gaap_CommitmentsAndContingenciesDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LeaseArrangementTypeAxis" xlink:label="us-gaap_LeaseArrangementTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LeaseArrangementTypeDomain" xlink:label="us-gaap_LeaseArrangementTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LeaseArrangementTypeDomain" xlink:label="us-gaap_LeaseArrangementTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LeaseholdImprovementsMember" xlink:label="us-gaap_LeaseholdImprovementsMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_LeaseArrangementTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LeaseArrangementTypeDomain" xlink:to="bcrx_BirminghamResearchFacilityMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LeaseArrangementTypeAxis" xlink:to="us-gaap_LeaseArrangementTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_LeaseholdImprovementsMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_BalanceSheetLocationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BalanceSheetLocationDomain" xlink:to="bcrx_LongTermLiabilitiesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LeaseArrangementTypeAxis" xlink:to="us-gaap_LeaseArrangementTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_CommitmentsAndContingenciesDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-5-stockholders-equity" xlink:href="bcrx-20151231.xsd#statement-note-5-stockholders-equity" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-5-stockholders-equity" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockholdersEquityNoteDisclosureTextBlock" xlink:label="us-gaap_StockholdersEquityNoteDisclosureTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_StockholdersEquityNoteDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-6-stockbased-compensation" xlink:href="bcrx-20151231.xsd#statement-note-6-stockbased-compensation" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-6-stockbased-compensation" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_August2013Member" xlink:label="bcrx_August2013Member" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_December2014Member" xlink:label="bcrx_December2014Member" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_EmployeeStockPurchasePlanMember" xlink:label="bcrx_EmployeeStockPurchasePlanMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_IncentivePlanMember" xlink:label="bcrx_IncentivePlanMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_NonEmployeeDirectorsMember" xlink:label="bcrx_NonEmployeeDirectorsMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_StockOptionsAndRestrictedStockMember" xlink:label="bcrx_StockOptionsAndRestrictedStockMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_Vest25PercentEachYearUntilFullyVestedMember" xlink:label="bcrx_Vest25PercentEachYearUntilFullyVestedMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_Vest50PercentEachYearUntilFullyVestedMember" xlink:label="bcrx_Vest50PercentEachYearUntilFullyVestedMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_VestUponSuccessfulCompletionOfSpecificDevelopmentMilestonesMember" xlink:label="bcrx_VestUponSuccessfulCompletionOfSpecificDevelopmentMilestonesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardDateAxis" xlink:label="us-gaap_AwardDateAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardDateDomain" xlink:label="us-gaap_AwardDateDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardDateDomain" xlink:label="us-gaap_AwardDateDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock" xlink:label="us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="us-gaap_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="us-gaap_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PerformanceSharesMember" xlink:label="us-gaap_PerformanceSharesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="us-gaap_PlanNameAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:label="us-gaap_TitleOfIndividualAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VestingAxis" xlink:label="us-gaap_VestingAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VestingDomain" xlink:label="us-gaap_VestingDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VestingDomain" xlink:label="us-gaap_VestingDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_PlanNameAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PlanNameDomain" xlink:to="bcrx_IncentivePlanMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PlanNameDomain" xlink:to="bcrx_EmployeeStockPurchasePlanMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_AwardTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_VestingAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="bcrx_Vest25PercentEachYearUntilFullyVestedMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="bcrx_StockOptionsAndRestrictedStockMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="bcrx_Vest50PercentEachYearUntilFullyVestedMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_PerformanceSharesMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="bcrx_VestUponSuccessfulCompletionOfSpecificDevelopmentMilestonesMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_AwardDateAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AwardDateDomain" xlink:to="bcrx_August2013Member" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardDateAxis" xlink:to="us-gaap_AwardDateDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AwardDateDomain" xlink:to="bcrx_December2014Member" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_TitleOfIndividualAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="bcrx_NonEmployeeDirectorsMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_TitleOfIndividualAxis" xlink:to="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardDateAxis" xlink:to="us-gaap_AwardDateDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TitleOfIndividualAxis" xlink:to="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-7-income-taxes" xlink:href="bcrx-20151231.xsd#statement-note-7-income-taxes" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-7-income-taxes" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DomesticCountryMember" xlink:label="us-gaap_DomesticCountryMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="us-gaap_IncomeTaxAuthorityAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxDisclosureTextBlock" xlink:label="us-gaap_IncomeTaxDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchMember" xlink:label="us-gaap_ResearchMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StateAndLocalJurisdictionMember" xlink:label="us-gaap_StateAndLocalJurisdictionMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardAxis" xlink:label="us-gaap_TaxCreditCarryforwardAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardNameDomain" xlink:label="us-gaap_TaxCreditCarryforwardNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardNameDomain" xlink:label="us-gaap_TaxCreditCarryforwardNameDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_IncomeTaxAuthorityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_DomesticCountryMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_StateAndLocalJurisdictionMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_TaxCreditCarryforwardAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TaxCreditCarryforwardNameDomain" xlink:to="us-gaap_ResearchMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_TaxCreditCarryforwardAxis" xlink:to="us-gaap_TaxCreditCarryforwardNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TaxCreditCarryforwardAxis" xlink:to="us-gaap_TaxCreditCarryforwardNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_IncomeTaxDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-8-employee-401k-plan" xlink:href="bcrx-20151231.xsd#statement-note-8-employee-401k-plan" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-8-employee-401k-plan" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PensionAndOtherPostretirementBenefitsDisclosureTextBlock" xlink:label="us-gaap_PensionAndOtherPostretirementBenefitsDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_PensionAndOtherPostretirementBenefitsDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-9-collaborative-and-other-research-and-development-contracts" xlink:href="bcrx-20151231.xsd#statement-note-9-collaborative-and-other-research-and-development-contracts" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-9-collaborative-and-other-research-and-development-contracts" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AECOMAndIRLMember" xlink:label="bcrx_AECOMAndIRLMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ASPRBARDAContractMember" xlink:label="bcrx_ASPRBARDAContractMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AdditionalDevelopmentOptionsMember" xlink:label="bcrx_AdditionalDevelopmentOptionsMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_BaseContractMember" xlink:label="bcrx_BaseContractMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CSLLimitedMember" xlink:label="bcrx_CSLLimitedMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ClinicalTrial2007Member" xlink:label="bcrx_ClinicalTrial2007Member" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_GreenCrossCorporationMember" xlink:label="bcrx_GreenCrossCorporationMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_MundipharmaMember" xlink:label="bcrx_MundipharmaMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_NationalInstituteOfAllergyAndInfectiousDiseasesMember" xlink:label="bcrx_NationalInstituteOfAllergyAndInfectiousDiseasesMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_UABMember" xlink:label="bcrx_UABMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_USDepartmentOfHealthAndHumanServicesMember" xlink:label="bcrx_USDepartmentOfHealthAndHumanServicesMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_UpfrontPaymentMember" xlink:label="bcrx_UpfrontPaymentMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CollaborativeArrangementDisclosureTextBlock" xlink:label="us-gaap_CollaborativeArrangementDisclosureTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CounterpartyNameAxis" xlink:label="us-gaap_CounterpartyNameAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeAxis" xlink:label="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="us-gaap_DeferredRevenueArrangementTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DisclosureTextBlockAbstract" xlink:label="us-gaap_DisclosureTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="us-gaap_TypeOfArrangementAxis" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:to="bcrx_USDepartmentOfHealthAndHumanServicesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:to="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_TypeOfArrangementAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_BaseContractMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_AdditionalDevelopmentOptionsMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_ASPRBARDAContractMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:to="bcrx_NationalInstituteOfAllergyAndInfectiousDiseasesMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_CounterpartyNameAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="bcrx_CSLLimitedMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_GreenCrossCorporationMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_MundipharmaMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:to="bcrx_UpfrontPaymentMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:to="us-gaap_DeferredRevenueArrangementTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:to="bcrx_ClinicalTrial2007Member" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_AECOMAndIRLMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:to="bcrx_UABMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:to="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:to="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DisclosureTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DisclosureTextBlockAbstract" xlink:to="us-gaap_CollaborativeArrangementDisclosureTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-significant-accounting-policies-policies" xlink:href="bcrx-20151231.xsd#statement-significant-accounting-policies-policies" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-significant-accounting-policies-policies" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AccruedExpensesPolicyTextBlock" xlink:label="bcrx_AccruedExpensesPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ConcentrationOfMarketRiskPolicyTextBlock" xlink:label="bcrx_ConcentrationOfMarketRiskPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CurrencyHedgeAgreementPolicyTextBlock" xlink:label="bcrx_CurrencyHedgeAgreementPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_DescriptionOfCompanyPolicyTextBlock" xlink:label="bcrx_DescriptionOfCompanyPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_InterestExpenseAndDeferredFinancingCostsPolicyTextBlock" xlink:label="bcrx_InterestExpenseAndDeferredFinancingCostsPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountingPoliciesAbstract" xlink:label="us-gaap_AccountingPoliciesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CashAndCashEquivalentsPolicyTextBlock" xlink:label="us-gaap_CashAndCashEquivalentsPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CashAndCashEquivalentsRestrictedCashAndCashEquivalentsPolicy" xlink:label="us-gaap_CashAndCashEquivalentsRestrictedCashAndCashEquivalentsPolicy" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ComprehensiveIncomePolicyPolicyTextBlock" xlink:label="us-gaap_ComprehensiveIncomePolicyPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConsolidationPolicyTextBlock" xlink:label="us-gaap_ConsolidationPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EarningsPerSharePolicyTextBlock" xlink:label="us-gaap_EarningsPerSharePolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxPolicyTextBlock" xlink:label="us-gaap_IncomeTaxPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IntangibleAssetsFiniteLivedPolicy" xlink:label="us-gaap_IntangibleAssetsFiniteLivedPolicy" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InventoryPolicyTextBlock" xlink:label="us-gaap_InventoryPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InvestmentPolicyTextBlock" xlink:label="us-gaap_InvestmentPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PolicyTextBlockAbstract" xlink:label="us-gaap_PolicyTextBlockAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PriorPeriodReclassificationAdjustmentDescription" xlink:label="us-gaap_PriorPeriodReclassificationAdjustmentDescription" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentPolicyTextBlock" xlink:label="us-gaap_PropertyPlantAndEquipmentPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReceivablesPolicyTextBlock" xlink:label="us-gaap_ReceivablesPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentExpensePolicy" xlink:label="us-gaap_ResearchAndDevelopmentExpensePolicy" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RevenueRecognitionPolicyTextBlock" xlink:label="us-gaap_RevenueRecognitionPolicyTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationOptionAndIncentivePlansPolicy" xlink:label="us-gaap_ShareBasedCompensationOptionAndIncentivePlansPolicy" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UseOfEstimates" xlink:label="us-gaap_UseOfEstimates" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PolicyTextBlockAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PolicyTextBlockAbstract" xlink:to="us-gaap_AccountingPoliciesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="bcrx_DescriptionOfCompanyPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_ConsolidationPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_PriorPeriodReclassificationAdjustmentDescription" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_CashAndCashEquivalentsPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_CashAndCashEquivalentsRestrictedCashAndCashEquivalentsPolicy" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_InvestmentPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_ReceivablesPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_InventoryPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_PropertyPlantAndEquipmentPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_IntangibleAssetsFiniteLivedPolicy" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="bcrx_AccruedExpensesPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_IncomeTaxPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_ComprehensiveIncomePolicyPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_RevenueRecognitionPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_ResearchAndDevelopmentExpensePolicy" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_ShareBasedCompensationOptionAndIncentivePlansPolicy" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="bcrx_InterestExpenseAndDeferredFinancingCostsPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="bcrx_CurrencyHedgeAgreementPolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="18" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_EarningsPerSharePolicyTextBlock" xlink:type="arc"/>
    <link:definitionArc order="19" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="us-gaap_UseOfEstimates" xlink:type="arc"/>
    <link:definitionArc order="20" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AccountingPoliciesAbstract" xlink:to="bcrx_ConcentrationOfMarketRiskPolicyTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-1-significant-accounting-policies-and-concentrations-of-risk-tables" xlink:href="bcrx-20151231.xsd#statement-note-1-significant-accounting-policies-and-concentrations-of-risk-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-1-significant-accounting-policies-and-concentrations-of-risk-tables" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AvailableForSaleSecuritiesDebtMaturitiesFairValueTableTextBlock" xlink:label="bcrx_AvailableForSaleSecuritiesDebtMaturitiesFairValueTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ScheduleOfReceivablesFromCollaborationsTableTextBlock" xlink:label="bcrx_ScheduleOfReceivablesFromCollaborationsTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ScheduleOfRevenuesFromCollaborationsTableTextBlock" xlink:label="bcrx_ScheduleOfRevenuesFromCollaborationsTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesTextBlock" xlink:label="us-gaap_AvailableForSaleSecuritiesTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfAccruedLiabilitiesTableTextBlock" xlink:label="us-gaap_ScheduleOfAccruedLiabilitiesTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfInventoryCurrentTableTextBlock" xlink:label="us-gaap_ScheduleOfInventoryCurrentTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_AvailableForSaleSecuritiesTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="bcrx_AvailableForSaleSecuritiesDebtMaturitiesFairValueTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="bcrx_ScheduleOfReceivablesFromCollaborationsTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfInventoryCurrentTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfAccruedLiabilitiesTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="bcrx_ScheduleOfRevenuesFromCollaborationsTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-2-furniture-and-equipment-tables" xlink:href="bcrx-20151231.xsd#statement-note-2-furniture-and-equipment-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-2-furniture-and-equipment-tables" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTextBlock" xlink:label="us-gaap_PropertyPlantAndEquipmentTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_PropertyPlantAndEquipmentTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-4-lease-obligations-and-other-contingencies-tables" xlink:href="bcrx-20151231.xsd#statement-note-4-lease-obligations-and-other-contingencies-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-4-lease-obligations-and-other-contingencies-tables" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfFutureMinimumRentalPaymentsForOperatingLeasesTableTextBlock" xlink:label="us-gaap_ScheduleOfFutureMinimumRentalPaymentsForOperatingLeasesTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfFutureMinimumRentalPaymentsForOperatingLeasesTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-6-stockbased-compensation-tables" xlink:href="bcrx-20151231.xsd#statement-note-6-stockbased-compensation-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-6-stockbased-compensation-tables" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationActivityTableTextBlock" xlink:label="us-gaap_ScheduleOfShareBasedCompensationActivityTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" xlink:label="us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock" xlink:label="us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAndExercisableTableTextBlock" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAndExercisableTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfShareBasedCompensationActivityTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfShareBasedPaymentAwardStockOptionsValuationAssumptionsTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAndExercisableTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfShareBasedCompensationStockOptionsActivityTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-7-income-taxes-tables" xlink:href="bcrx-20151231.xsd#statement-note-7-income-taxes-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-7-income-taxes-tables" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfDeferredTaxAssetsAndLiabilitiesTableTextBlock" xlink:label="us-gaap_ScheduleOfDeferredTaxAssetsAndLiabilitiesTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEffectiveIncomeTaxRateReconciliationTableTextBlock" xlink:label="us-gaap_ScheduleOfEffectiveIncomeTaxRateReconciliationTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfUnrecognizedTaxBenefitsRollForwardTableTextBlock" xlink:label="us-gaap_ScheduleOfUnrecognizedTaxBenefitsRollForwardTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfEffectiveIncomeTaxRateReconciliationTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfUnrecognizedTaxBenefitsRollForwardTableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfDeferredTaxAssetsAndLiabilitiesTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-10-quarterly-financial-information-unaudited-tables" xlink:href="bcrx-20151231.xsd#statement-note-10-quarterly-financial-information-unaudited-tables" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-10-quarterly-financial-information-unaudited-tables" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfQuarterlyFinancialInformationTableTextBlock" xlink:label="us-gaap_ScheduleOfQuarterlyFinancialInformationTableTextBlock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TableTextBlock" xlink:label="us-gaap_TableTextBlock" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TableTextBlock" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TableTextBlock" xlink:to="us-gaap_ScheduleOfQuarterlyFinancialInformationTableTextBlock" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-1-significant-accounting-policies-and-concentrations-of-risk-details-textual" xlink:href="bcrx-20151231.xsd#statement-note-1-significant-accounting-policies-and-concentrations-of-risk-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-1-significant-accounting-policies-and-concentrations-of-risk-details-textual" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AgreementMember" xlink:label="bcrx_AgreementMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AveragematurityForPortfolioInvestments" xlink:label="bcrx_AveragematurityForPortfolioInvestments" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AveragematurityPeriodOfHighQualityMarketableSecurities" xlink:label="bcrx_AveragematurityPeriodOfHighQualityMarketableSecurities" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CSLMember" xlink:label="bcrx_CSLMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CollateralForCreditMember" xlink:label="bcrx_CollateralForCreditMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ContingentUponEuMarketingApprovalMember" xlink:label="bcrx_ContingentUponEuMarketingApprovalMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CurrencyHedgeAgreementMember" xlink:label="bcrx_CurrencyHedgeAgreementMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_JPRRoyaltySubLLCMember" xlink:label="bcrx_JPRRoyaltySubLLCMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_LaboratoryEquipmentOfficeEquipmentAndSoftwareMember" xlink:label="bcrx_LaboratoryEquipmentOfficeEquipmentAndSoftwareMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_LongtermInvestmentMaturityMinimum" xlink:label="bcrx_LongtermInvestmentMaturityMinimum" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_MaturityOfInvestments" xlink:label="bcrx_MaturityOfInvestments" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_MaturityPeriodOfHighQualityMarketablesecurities" xlink:label="bcrx_MaturityPeriodOfHighQualityMarketablesecurities" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_MaturityperiodOfShortTermInvestment" xlink:label="bcrx_MaturityperiodOfShortTermInvestment" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_MilestonePaymentMaximum" xlink:label="bcrx_MilestonePaymentMaximum" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_PhaRMANotesMember" xlink:label="bcrx_PhaRMANotesMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RAPIVABMember" xlink:label="bcrx_RAPIVABMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RegulatorySupportRevenueForCanadianAndEUMarketingApprovalsPortionRecognizedRatablyOverExpectedPeriodOfInvolvementMember" xlink:label="bcrx_RegulatorySupportRevenueForCanadianAndEUMarketingApprovalsPortionRecognizedRatablyOverExpectedPeriodOfInvolvementMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RevenueFromSaleOfInventoryToBeRecognizedWhenTheInventoryTransferIsCompleteMember" xlink:label="bcrx_RevenueFromSaleOfInventoryToBeRecognizedWhenTheInventoryTransferIsCompleteMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RevenuerecognitionRoyaltyAndMilestoneRevenueRecognized" xlink:label="bcrx_RevenuerecognitionRoyaltyAndMilestoneRevenueRecognized" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RoyaltyReceivableMember" xlink:label="bcrx_RoyaltyReceivableMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_SecuritiesOffered" xlink:label="bcrx_SecuritiesOffered" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AdvertisingExpense" xlink:label="us-gaap_AdvertisingExpense" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AmortizationOfFinancingCosts" xlink:label="us-gaap_AmortizationOfFinancingCosts" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BuildingMember" xlink:label="us-gaap_BuildingMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CertificatesOfDepositMember" xlink:label="us-gaap_CertificatesOfDepositMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CollateralAlreadyPostedAggregateFairValue" xlink:label="us-gaap_CollateralAlreadyPostedAggregateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ComputerEquipmentMember" xlink:label="us-gaap_ComputerEquipmentMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="us-gaap_ConcentrationRiskBenchmarkDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:label="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskByTypeAxis" xlink:label="us-gaap_ConcentrationRiskByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskPercentage1" xlink:label="us-gaap_ConcentrationRiskPercentage1" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="us-gaap_ConcentrationRiskTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="us-gaap_ConcentrationRiskTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CounterpartyNameAxis" xlink:label="us-gaap_CounterpartyNameAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CustomerConcentrationRiskMember" xlink:label="us-gaap_CustomerConcentrationRiskMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueAdditions" xlink:label="us-gaap_DeferredRevenueAdditions" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeAxis" xlink:label="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="us-gaap_DeferredRevenueArrangementTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeGainOnDerivative" xlink:label="us-gaap_DerivativeGainOnDerivative" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeLossOnDerivative" xlink:label="us-gaap_DerivativeLossOnDerivative" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForeignExchangeContractMember" xlink:label="us-gaap_ForeignExchangeContractMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="us-gaap_FurnitureAndFixturesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationAxis" xlink:label="us-gaap_HedgingDesignationAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="us-gaap_HedgingDesignationDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="us-gaap_HedgingDesignationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InterestExpense" xlink:label="us-gaap_InterestExpense" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InvestmentsAndCash" xlink:label="us-gaap_InvestmentsAndCash" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongtermDebtTypeAxis" xlink:label="us-gaap_LongtermDebtTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="us-gaap_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="us-gaap_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NondesignatedMember" xlink:label="us-gaap_NondesignatedMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProceedsFromLicenseFeesReceived" xlink:label="us-gaap_ProceedsFromLicenseFeesReceived" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductOrServiceAxis" xlink:label="us-gaap_ProductOrServiceAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="us-gaap_ProductsAndServicesDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProductsAndServicesDomain" xlink:label="us-gaap_ProductsAndServicesDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:label="us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax" xlink:label="us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalents" xlink:label="us-gaap_RestrictedCashAndCashEquivalents" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_Revenues" xlink:label="us-gaap_Revenues" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueProductLineMember" xlink:label="us-gaap_SalesRevenueProductLineMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="us-gaap_TypeOfArrangementAxis" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="bcrx_CurrencyHedgeAgreementMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_EntityDomain" xlink:to="bcrx_JPRRoyaltySubLLCMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:to="us-gaap_CertificatesOfDepositMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:to="bcrx_RoyaltyReceivableMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:to="bcrx_CollateralForCreditMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ComputerEquipmentMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="bcrx_LaboratoryEquipmentOfficeEquipmentAndSoftwareMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_FurnitureAndFixturesMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_BuildingMember" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_TypeOfArrangementAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_AgreementMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember-default" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_CounterpartyNameAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="bcrx_CSLMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_ProductOrServiceAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ProductsAndServicesDomain" xlink:to="bcrx_RAPIVABMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ProductOrServiceAxis" xlink:to="us-gaap_ProductsAndServicesDomain-default" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScenarioUnspecifiedDomain" xlink:to="bcrx_ContingentUponEuMarketingApprovalMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:to="bcrx_RevenueFromSaleOfInventoryToBeRecognizedWhenTheInventoryTransferIsCompleteMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:to="us-gaap_DeferredRevenueArrangementTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:to="bcrx_RegulatorySupportRevenueForCanadianAndEUMarketingApprovalsPortionRecognizedRatablyOverExpectedPeriodOfInvolvementMember" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_LongtermDebtTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="bcrx_PhaRMANotesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="us-gaap_ForeignExchangeContractMember" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_HedgingDesignationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_HedgingDesignationDomain" xlink:to="us-gaap_NondesignatedMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_HedgingDesignationAxis" xlink:to="us-gaap_HedgingDesignationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_ConcentrationRiskByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ConcentrationRiskTypeDomain" xlink:to="us-gaap_CustomerConcentrationRiskMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ConcentrationRiskByTypeAxis" xlink:to="us-gaap_ConcentrationRiskTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:to="us-gaap_SalesRevenueProductLineMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:to="us-gaap_ConcentrationRiskBenchmarkDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_HedgingDesignationAxis" xlink:to="us-gaap_HedgingDesignationDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ProductOrServiceAxis" xlink:to="us-gaap_ProductsAndServicesDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:to="us-gaap_ConcentrationRiskBenchmarkDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsAxis" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ConcentrationRiskByTypeAxis" xlink:to="us-gaap_ConcentrationRiskTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:to="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CollateralAlreadyPostedAggregateFairValue" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AmortizationOfFinancingCosts" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InvestmentsAndCash" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_SecuritiesOffered" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_RevenuerecognitionRoyaltyAndMilestoneRevenueRecognized" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_MaturityOfInvestments" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_RestrictedCashAndCashEquivalents" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_MaturityPeriodOfHighQualityMarketablesecurities" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_AveragematurityPeriodOfHighQualityMarketableSecurities" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_MaturityperiodOfShortTermInvestment" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_LongtermInvestmentMaturityMinimum" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentUsefulLife" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ProceedsFromLicenseFeesReceived" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_Revenues" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredRevenueAdditions" xlink:type="arc"/>
    <link:definitionArc order="18" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_MilestonePaymentMaximum" xlink:type="arc"/>
    <link:definitionArc order="19" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AdvertisingExpense" xlink:type="arc"/>
    <link:definitionArc order="20" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InterestExpense" xlink:type="arc"/>
    <link:definitionArc order="21" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeLossOnDerivative" xlink:type="arc"/>
    <link:definitionArc order="22" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeGainOnDerivative" xlink:type="arc"/>
    <link:definitionArc order="23" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:type="arc"/>
    <link:definitionArc order="24" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ConcentrationRiskPercentage1" xlink:type="arc"/>
    <link:definitionArc order="25" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_AveragematurityForPortfolioInvestments" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-2-furniture-and-equipment-details-textual" xlink:href="bcrx-20151231.xsd#statement-note-2-furniture-and-equipment-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-2-furniture-and-equipment-details-textual" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="us-gaap_DepreciationDepletionAndAmortization" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DepreciationDepletionAndAmortization" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-3-royalty-monetization-details-textual" xlink:href="bcrx-20151231.xsd#statement-note-3-royalty-monetization-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-3-royalty-monetization-details-textual" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CurrencyDomain" xlink:label="bcrx_CurrencyDomain" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CurrencyDomain" xlink:label="bcrx_CurrencyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CurrencyHedgeAgreementMember" xlink:label="bcrx_CurrencyHedgeAgreementMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_Interestreserve" xlink:label="bcrx_Interestreserve" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_JPRRoyaltySubLLCMember" xlink:label="bcrx_JPRRoyaltySubLLCMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_JapanYenMember" xlink:label="bcrx_JapanYenMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_MaximumAmountOfCollateralRequiredToPost" xlink:label="bcrx_MaximumAmountOfCollateralRequiredToPost" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_PercentageOfCarryingAmountInExcessOfFairValue1" xlink:label="bcrx_PercentageOfCarryingAmountInExcessOfFairValue1" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_PhaRMANotesMember" xlink:label="bcrx_PhaRMANotesMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_PrivatePlacementOfSeniorSecuredNotes" xlink:label="bcrx_PrivatePlacementOfSeniorSecuredNotes" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RequiredForeignCurrencyHedgePerDollar" xlink:label="bcrx_RequiredForeignCurrencyHedgePerDollar" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RevenueRecognitionRoyaltyAndMilestoneRevenueRecognizedNet" xlink:label="bcrx_RevenueRecognitionRoyaltyAndMilestoneRevenueRecognizedNet" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RevenuerecognitionRoyaltyAndMilestoneRevenueRecognized" xlink:label="bcrx_RevenuerecognitionRoyaltyAndMilestoneRevenueRecognized" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RoyaltyMonetizationMember" xlink:label="bcrx_RoyaltyMonetizationMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_Shortfall2013Member" xlink:label="bcrx_Shortfall2013Member" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_TransactionCosts" xlink:label="bcrx_TransactionCosts" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="dei_EntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:label="us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CollateralAlreadyPostedAggregateFairValue" xlink:label="us-gaap_CollateralAlreadyPostedAggregateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CurrencyAxis" xlink:label="us-gaap_CurrencyAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentAxis" xlink:label="us-gaap_DebtInstrumentAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DebtInstrumentNameDomain" xlink:label="us-gaap_DebtInstrumentNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeContractTypeDomain" xlink:label="us-gaap_DerivativeContractTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeForwardExchangeRate1" xlink:label="us-gaap_DerivativeForwardExchangeRate1" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeGainOnDerivative" xlink:label="us-gaap_DerivativeGainOnDerivative" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeInstrumentRiskAxis" xlink:label="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeLossOnDerivative" xlink:label="us-gaap_DerivativeLossOnDerivative" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="us-gaap_FairValueInputsLevel2Member" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ForeignExchangeContractMember" xlink:label="us-gaap_ForeignExchangeContractMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationAxis" xlink:label="us-gaap_HedgingDesignationAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="us-gaap_HedgingDesignationDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_HedgingDesignationDomain" xlink:label="us-gaap_HedgingDesignationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InterestPaid" xlink:label="us-gaap_InterestPaid" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InterestPayableCurrentAndNoncurrent" xlink:label="us-gaap_InterestPayableCurrentAndNoncurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:label="us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:label="us-gaap_LegalEntityTypeOfCounterpartyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NondesignatedMember" xlink:label="us-gaap_NondesignatedMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NotesPayableFairValueDisclosure" xlink:label="us-gaap_NotesPayableFairValueDisclosure" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCommitmentsAxis" xlink:label="us-gaap_OtherCommitmentsAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCommitmentsDomain" xlink:label="us-gaap_OtherCommitmentsDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCommitmentsDomain" xlink:label="us-gaap_OtherCommitmentsDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsForProceedsFromHedgeInvestingActivities" xlink:label="us-gaap_PaymentsForProceedsFromHedgeInvestingActivities" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_DebtInstrumentAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DebtInstrumentNameDomain" xlink:to="bcrx_PhaRMANotesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:to="bcrx_JPRRoyaltySubLLCMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:to="us-gaap_LegalEntityTypeOfCounterpartyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_DerivativeInstrumentRiskAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="bcrx_CurrencyHedgeAgreementMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="dei_LegalEntityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="dei_EntityDomain" xlink:to="bcrx_JPRRoyaltySubLLCMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_OtherCommitmentsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_OtherCommitmentsDomain" xlink:to="bcrx_RoyaltyMonetizationMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_OtherCommitmentsAxis" xlink:to="us-gaap_OtherCommitmentsDomain-default" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ScenarioUnspecifiedDomain" xlink:to="bcrx_Shortfall2013Member" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueInputsLevel2Member" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_CurrencyAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="bcrx_CurrencyDomain" xlink:to="bcrx_JapanYenMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CurrencyAxis" xlink:to="bcrx_CurrencyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DerivativeContractTypeDomain" xlink:to="us-gaap_ForeignExchangeContractMember" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_HedgingDesignationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_HedgingDesignationDomain" xlink:to="us-gaap_NondesignatedMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_HedgingDesignationAxis" xlink:to="us-gaap_HedgingDesignationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="dei_LegalEntityAxis" xlink:to="dei_EntityDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_HedgingDesignationAxis" xlink:to="us-gaap_HedgingDesignationDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CurrencyAxis" xlink:to="bcrx_CurrencyDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:to="us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DerivativeInstrumentRiskAxis" xlink:to="us-gaap_DerivativeContractTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DebtInstrumentAxis" xlink:to="us-gaap_DebtInstrumentNameDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_OtherCommitmentsAxis" xlink:to="us-gaap_OtherCommitmentsDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CollateralAlreadyPostedAggregateFairValue" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_PercentageOfCarryingAmountInExcessOfFairValue1" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_RevenuerecognitionRoyaltyAndMilestoneRevenueRecognized" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_RevenueRecognitionRoyaltyAndMilestoneRevenueRecognizedNet" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_TransactionCosts" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_Interestreserve" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_PrivatePlacementOfSeniorSecuredNotes" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DebtInstrumentInterestRateStatedPercentage" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InterestPaid" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InterestPayableCurrentAndNoncurrent" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NotesPayableFairValueDisclosure" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeForwardExchangeRate1" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PaymentsForProceedsFromHedgeInvestingActivities" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_RequiredForeignCurrencyHedgePerDollar" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeLossOnDerivative" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DerivativeGainOnDerivative" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_MaximumAmountOfCollateralRequiredToPost" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-4-lease-obligations-and-other-contingencies-details-textual" xlink:href="bcrx-20151231.xsd#statement-note-4-lease-obligations-and-other-contingencies-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-4-lease-obligations-and-other-contingencies-details-textual" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_BirminghamResearchFacilityMember" xlink:label="bcrx_BirminghamResearchFacilityMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_LeaseFinancingObligationNetOfCurrent" xlink:label="bcrx_LeaseFinancingObligationNetOfCurrent" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_LongTermLiabilitiesMember" xlink:label="bcrx_LongTermLiabilitiesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AreaOfRealEstateProperty" xlink:label="us-gaap_AreaOfRealEstateProperty" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="us-gaap_BalanceSheetLocationAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="us-gaap_BalanceSheetLocationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LeaseArrangementTypeAxis" xlink:label="us-gaap_LeaseArrangementTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LeaseArrangementTypeDomain" xlink:label="us-gaap_LeaseArrangementTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LeaseArrangementTypeDomain" xlink:label="us-gaap_LeaseArrangementTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LeaseholdImprovementsMember" xlink:label="us-gaap_LeaseholdImprovementsMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDue" xlink:label="us-gaap_OperatingLeasesFutureMinimumPaymentsDue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingLeasesRentExpenseNet" xlink:label="us-gaap_OperatingLeasesRentExpenseNet" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="us-gaap_PropertyPlantAndEquipmentGross" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_LeaseArrangementTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LeaseArrangementTypeDomain" xlink:to="bcrx_BirminghamResearchFacilityMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_LeaseArrangementTypeAxis" xlink:to="us-gaap_LeaseArrangementTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_LeaseholdImprovementsMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_BalanceSheetLocationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_BalanceSheetLocationDomain" xlink:to="bcrx_LongTermLiabilitiesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_BalanceSheetLocationAxis" xlink:to="us-gaap_BalanceSheetLocationDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_LeaseArrangementTypeAxis" xlink:to="us-gaap_LeaseArrangementTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AreaOfRealEstateProperty" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeasesFutureMinimumPaymentsDue" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeasesRentExpenseNet" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentGross" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_LeaseFinancingObligationNetOfCurrent" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-5-stockholders-equity-details-textual" xlink:href="bcrx-20151231.xsd#statement-note-5-stockholders-equity-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-5-stockholders-equity-details-textual" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CommonStockToBeIssuedValue" xlink:label="bcrx_CommonStockToBeIssuedValue" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_MaximumAggregateOfferingPrice" xlink:label="bcrx_MaximumAggregateOfferingPrice" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RemainingAggregateOfferingPrice" xlink:label="bcrx_RemainingAggregateOfferingPrice" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_StockIssuedDuringPeriodSharesForOverAllotmentOptionExercisedByUnderwriter" xlink:label="bcrx_StockIssuedDuringPeriodSharesForOverAllotmentOptionExercisedByUnderwriter" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommonStockValue" xlink:label="us-gaap_CommonStockValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlansIncludingStockOptions" xlink:label="us-gaap_ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlansIncludingStockOptions" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProceedsFromIssuanceOrSaleOfEquity" xlink:label="us-gaap_ProceedsFromIssuanceOrSaleOfEquity" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharePrice" xlink:label="us-gaap_SharePrice" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_MaximumAggregateOfferingPrice" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ProceedsFromIssuanceOrSaleOfEquity" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_RemainingAggregateOfferingPrice" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharePrice" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_StockIssuedDuringPeriodSharesForOverAllotmentOptionExercisedByUnderwriter" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_CommonStockToBeIssuedValue" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CommonStockValue" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ProceedsFromIssuanceOfSharesUnderIncentiveAndShareBasedCompensationPlansIncludingStockOptions" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-6-stockbased-compensation-details-textual" xlink:href="bcrx-20151231.xsd#statement-note-6-stockbased-compensation-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-6-stockbased-compensation-details-textual" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_August2013Member" xlink:label="bcrx_August2013Member" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_December2014Member" xlink:label="bcrx_December2014Member" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostExpectedToBeRecognizedForRemainderOfFiscalYear" xlink:label="bcrx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostExpectedToBeRecognizedForRemainderOfFiscalYear" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostExpectedToBeRecognizedYearFour" xlink:label="bcrx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostExpectedToBeRecognizedYearFour" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostExpectedToBeRecognizedYearThree" xlink:label="bcrx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostExpectedToBeRecognizedYearThree" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostExpectedToBeRecognizedYearTwo" xlink:label="bcrx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostExpectedToBeRecognizedYearTwo" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_EmployeeStockPurchasePlanMember" xlink:label="bcrx_EmployeeStockPurchasePlanMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_IncentivePlanMember" xlink:label="bcrx_IncentivePlanMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_NonEmployeeDirectorsMember" xlink:label="bcrx_NonEmployeeDirectorsMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_NumberOfMilestonesAchieved" xlink:label="bcrx_NumberOfMilestonesAchieved" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_NumberOfStockbasedCompensationPlans" xlink:label="bcrx_NumberOfStockbasedCompensationPlans" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_PercentageOfCommonStockSharesBeginning" xlink:label="bcrx_PercentageOfCommonStockSharesBeginning" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_PercentageOfCommonStockSharesEnding" xlink:label="bcrx_PercentageOfCommonStockSharesEnding" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_PercentageOfSalaryToPurchaseCommonStockMaximum" xlink:label="bcrx_PercentageOfSalaryToPurchaseCommonStockMaximum" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ShareBasedCompensationArrangementBySharbasedPaymentAwardMaximumNumberOfSharesPerEmployeeAmount" xlink:label="bcrx_ShareBasedCompensationArrangementBySharbasedPaymentAwardMaximumNumberOfSharesPerEmployeeAmount" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ShareBasedCompensationArrangementByShareBasedPaymentAwardFirstVestingPeriodAfterGrantDate" xlink:label="bcrx_ShareBasedCompensationArrangementByShareBasedPaymentAwardFirstVestingPeriodAfterGrantDate" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_SharebasedCompensationArrangementBySharbasedPaymentAwardAwardsUnvestedPercentage" xlink:label="bcrx_SharebasedCompensationArrangementBySharbasedPaymentAwardAwardsUnvestedPercentage" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_StockOptionsAndRestrictedStockMember" xlink:label="bcrx_StockOptionsAndRestrictedStockMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_Vest25PercentEachYearUntilFullyVestedMember" xlink:label="bcrx_Vest25PercentEachYearUntilFullyVestedMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_Vest50PercentEachYearUntilFullyVestedMember" xlink:label="bcrx_Vest50PercentEachYearUntilFullyVestedMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_VestUponSuccessfulCompletionOfSpecificDevelopmentMilestonesMember" xlink:label="bcrx_VestUponSuccessfulCompletionOfSpecificDevelopmentMilestonesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardDateAxis" xlink:label="us-gaap_AwardDateAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardDateDomain" xlink:label="us-gaap_AwardDateDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardDateDomain" xlink:label="us-gaap_AwardDateDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="us-gaap_AwardTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="us-gaap_EmployeeStockOptionMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeStockOwnershipPlanESOPWeightedAveragePurchasePriceOfSharesPurchased" xlink:label="us-gaap_EmployeeStockOwnershipPlanESOPWeightedAveragePurchasePriceOfSharesPurchased" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="us-gaap_MaximumMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="us-gaap_MinimumMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PerformanceSharesMember" xlink:label="us-gaap_PerformanceSharesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="us-gaap_PlanNameAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:label="us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TitleOfIndividualAxis" xlink:label="us-gaap_TitleOfIndividualAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:label="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VestingAxis" xlink:label="us-gaap_VestingAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VestingDomain" xlink:label="us-gaap_VestingDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_VestingDomain" xlink:label="us-gaap_VestingDomain-default" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_PlanNameAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PlanNameDomain" xlink:to="bcrx_IncentivePlanMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PlanNameDomain" xlink:to="bcrx_EmployeeStockPurchasePlanMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_AwardTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_EmployeeStockOptionMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_VestingAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="bcrx_Vest25PercentEachYearUntilFullyVestedMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="bcrx_StockOptionsAndRestrictedStockMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="bcrx_Vest50PercentEachYearUntilFullyVestedMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_PerformanceSharesMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_VestingDomain" xlink:to="bcrx_VestUponSuccessfulCompletionOfSpecificDevelopmentMilestonesMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_AwardDateAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AwardDateDomain" xlink:to="bcrx_August2013Member" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AwardDateAxis" xlink:to="us-gaap_AwardDateDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AwardDateDomain" xlink:to="bcrx_December2014Member" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_TitleOfIndividualAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:to="bcrx_NonEmployeeDirectorsMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_TitleOfIndividualAxis" xlink:to="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_RangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MinimumMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_MaximumMember" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardDateAxis" xlink:to="us-gaap_AwardDateDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AwardTypeAxis" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TitleOfIndividualAxis" xlink:to="us-gaap_TitleOfIndividualWithRelationshipToEntityDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_VestingAxis" xlink:to="us-gaap_VestingDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_NumberOfStockbasedCompensationPlans" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_NumberOfMilestonesAchieved" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_SharebasedCompensationArrangementBySharbasedPaymentAwardAwardsUnvestedPercentage" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_ShareBasedCompensationArrangementByShareBasedPaymentAwardFirstVestingPeriodAfterGrantDate" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1" xlink:type="arc"/>
    <link:definitionArc order="18" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:type="arc"/>
    <link:definitionArc order="19" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostExpectedToBeRecognizedForRemainderOfFiscalYear" xlink:type="arc"/>
    <link:definitionArc order="20" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostExpectedToBeRecognizedYearTwo" xlink:type="arc"/>
    <link:definitionArc order="21" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostExpectedToBeRecognizedYearThree" xlink:type="arc"/>
    <link:definitionArc order="22" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_EmployeeServiceShareBasedCompensationNonvestedAwardsCompensationCostExpectedToBeRecognizedYearFour" xlink:type="arc"/>
    <link:definitionArc order="23" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized" xlink:type="arc"/>
    <link:definitionArc order="24" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:type="arc"/>
    <link:definitionArc order="25" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_PercentageOfSalaryToPurchaseCommonStockMaximum" xlink:type="arc"/>
    <link:definitionArc order="26" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_PercentageOfCommonStockSharesBeginning" xlink:type="arc"/>
    <link:definitionArc order="27" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_PercentageOfCommonStockSharesEnding" xlink:type="arc"/>
    <link:definitionArc order="28" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumNumberOfSharesPerEmployee" xlink:type="arc"/>
    <link:definitionArc order="29" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_ShareBasedCompensationArrangementBySharbasedPaymentAwardMaximumNumberOfSharesPerEmployeeAmount" xlink:type="arc"/>
    <link:definitionArc order="30" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:type="arc"/>
    <link:definitionArc order="31" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EmployeeStockOwnershipPlanESOPWeightedAveragePurchasePriceOfSharesPurchased" xlink:type="arc"/>
    <link:definitionArc order="32" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-7-income-taxes-details-textual" xlink:href="bcrx-20151231.xsd#statement-note-7-income-taxes-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-7-income-taxes-details-textual" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DomesticCountryMember" xlink:label="us-gaap_DomesticCountryMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis" xlink:label="us-gaap_IncomeTaxAuthorityAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain" xlink:label="us-gaap_IncomeTaxAuthorityDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:label="us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingLossCarryforwards" xlink:label="us-gaap_OperatingLossCarryforwards" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchMember" xlink:label="us-gaap_ResearchMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StateAndLocalJurisdictionMember" xlink:label="us-gaap_StateAndLocalJurisdictionMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardAmount" xlink:label="us-gaap_TaxCreditCarryforwardAmount" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardAxis" xlink:label="us-gaap_TaxCreditCarryforwardAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardNameDomain" xlink:label="us-gaap_TaxCreditCarryforwardNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxCreditCarryforwardNameDomain" xlink:label="us-gaap_TaxCreditCarryforwardNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued" xlink:label="us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_IncomeTaxAuthorityAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_DomesticCountryMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeTaxAuthorityDomain" xlink:to="us-gaap_StateAndLocalJurisdictionMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_TaxCreditCarryforwardAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_TaxCreditCarryforwardNameDomain" xlink:to="us-gaap_ResearchMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_TaxCreditCarryforwardAxis" xlink:to="us-gaap_TaxCreditCarryforwardNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeTaxAuthorityAxis" xlink:to="us-gaap_IncomeTaxAuthorityDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TaxCreditCarryforwardAxis" xlink:to="us-gaap_TaxCreditCarryforwardNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLossCarryforwards" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TaxCreditCarryforwardAmount" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitRealizedFromExerciseOfStockOptions" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-8-employee-401k-plan-details-textual" xlink:href="bcrx-20151231.xsd#statement-note-8-employee-401k-plan-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-8-employee-401k-plan-details-textual" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount" xlink:label="us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DefinedContributionPlanEmployerDiscretionaryContributionAmount" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-9-collaborative-and-other-research-and-development-contracts-details-textual" xlink:href="bcrx-20151231.xsd#statement-note-9-collaborative-and-other-research-and-development-contracts-details-textual" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-9-collaborative-and-other-research-and-development-contracts-details-textual" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AECOMAndIRLMember" xlink:label="bcrx_AECOMAndIRLMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ASPRBARDAContractMember" xlink:label="bcrx_ASPRBARDAContractMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AdditionalDevelopmentOptionsMember" xlink:label="bcrx_AdditionalDevelopmentOptionsMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AdvanceNoticePeriodForTerminationOfAgreement" xlink:label="bcrx_AdvanceNoticePeriodForTerminationOfAgreement" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AnnualLicenseFeeMaximum" xlink:label="bcrx_AnnualLicenseFeeMaximum" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AnnualLicenseFeeMinimum" xlink:label="bcrx_AnnualLicenseFeeMinimum" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_BaseContractMember" xlink:label="bcrx_BaseContractMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CSLLimitedMember" xlink:label="bcrx_CSLLimitedMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ClinicalTrial2007Member" xlink:label="bcrx_ClinicalTrial2007Member" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CollaborationAgreementAdditionalPaymentsReceived" xlink:label="bcrx_CollaborationAgreementAdditionalPaymentsReceived" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CollaborativeAgreementAdditionalContractValue" xlink:label="bcrx_CollaborativeAgreementAdditionalContractValue" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CollaborativeAgreementAdjustedContractValue" xlink:label="bcrx_CollaborativeAgreementAdjustedContractValue" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CollaborativeAgreementContractValue" xlink:label="bcrx_CollaborativeAgreementContractValue" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CollaborativeAgreementPeriodOfcontract" xlink:label="bcrx_CollaborativeAgreementPeriodOfcontract" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ExpectedReceivableFromAwardsForResearchAndDevelopmentContracts" xlink:label="bcrx_ExpectedReceivableFromAwardsForResearchAndDevelopmentContracts" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_GreenCrossCorporationMember" xlink:label="bcrx_GreenCrossCorporationMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_MilestonePaymentMaximum" xlink:label="bcrx_MilestonePaymentMaximum" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_MilestonePaymentMinimum" xlink:label="bcrx_MilestonePaymentMinimum" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_MundipharmaMember" xlink:label="bcrx_MundipharmaMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_NationalInstituteOfAllergyAndInfectiousDiseasesMember" xlink:label="bcrx_NationalInstituteOfAllergyAndInfectiousDiseasesMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_PaymentsForModificationOfLicenseAgreement" xlink:label="bcrx_PaymentsForModificationOfLicenseAgreement" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_PeriodOfAgreement" xlink:label="bcrx_PeriodOfAgreement" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_PotentialMilestonePaymentsReceivable" xlink:label="bcrx_PotentialMilestonePaymentsReceivable" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ProceedsFromAwardsForResearchAndDevelopmentContracts" xlink:label="bcrx_ProceedsFromAwardsForResearchAndDevelopmentContracts" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RenewablePeriodOfAgreement" xlink:label="bcrx_RenewablePeriodOfAgreement" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_RoyaltyTerm" xlink:label="bcrx_RoyaltyTerm" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_StockIssuedDuringPeriodValueForModificationOfLicenseAgreement" xlink:label="bcrx_StockIssuedDuringPeriodValueForModificationOfLicenseAgreement" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_UABMember" xlink:label="bcrx_UABMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_USDepartmentOfHealthAndHumanServicesMember" xlink:label="bcrx_USDepartmentOfHealthAndHumanServicesMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_UpfrontPaymentMember" xlink:label="bcrx_UpfrontPaymentMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_UpfrontPaymentsReceivableAmount" xlink:label="bcrx_UpfrontPaymentsReceivableAmount" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CounterpartyNameAxis" xlink:label="us-gaap_CounterpartyNameAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenue" xlink:label="us-gaap_DeferredRevenue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeAxis" xlink:label="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="us-gaap_DeferredRevenueArrangementTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GovernmentContractReceivable" xlink:label="us-gaap_GovernmentContractReceivable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProceedsFromLicenseFeesReceived" xlink:label="us-gaap_ProceedsFromLicenseFeesReceived" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="us-gaap_TypeOfArrangementAxis" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:to="bcrx_USDepartmentOfHealthAndHumanServicesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:to="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_TypeOfArrangementAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_BaseContractMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_AdditionalDevelopmentOptionsMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_ASPRBARDAContractMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:to="bcrx_NationalInstituteOfAllergyAndInfectiousDiseasesMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_CounterpartyNameAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="bcrx_CSLLimitedMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_GreenCrossCorporationMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_MundipharmaMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:to="bcrx_UpfrontPaymentMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:to="us-gaap_DeferredRevenueArrangementTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:to="bcrx_ClinicalTrial2007Member" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:to="bcrx_AECOMAndIRLMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:to="bcrx_UABMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_ArrangementsAndNonarrangementTransactionsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:to="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersByTypeAxis" xlink:to="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_CollaborativeAgreementContractValue" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_CollaborativeAgreementAdditionalContractValue" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_CollaborativeAgreementAdjustedContractValue" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_GovernmentContractReceivable" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_ProceedsFromAwardsForResearchAndDevelopmentContracts" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_ExpectedReceivableFromAwardsForResearchAndDevelopmentContracts" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_CollaborativeAgreementPeriodOfcontract" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_CollaborationAgreementAdditionalPaymentsReceived" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ProceedsFromLicenseFeesReceived" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_MilestonePaymentMaximum" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_RoyaltyTerm" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_UpfrontPaymentsReceivableAmount" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredRevenue" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_PotentialMilestonePaymentsReceivable" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_MilestonePaymentMinimum" xlink:type="arc"/>
    <link:definitionArc order="16" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_AnnualLicenseFeeMinimum" xlink:type="arc"/>
    <link:definitionArc order="17" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_AnnualLicenseFeeMaximum" xlink:type="arc"/>
    <link:definitionArc order="18" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_AdvanceNoticePeriodForTerminationOfAgreement" xlink:type="arc"/>
    <link:definitionArc order="19" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_StockIssuedDuringPeriodValueForModificationOfLicenseAgreement" xlink:type="arc"/>
    <link:definitionArc order="20" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_PaymentsForModificationOfLicenseAgreement" xlink:type="arc"/>
    <link:definitionArc order="21" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_PeriodOfAgreement" xlink:type="arc"/>
    <link:definitionArc order="22" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_RenewablePeriodOfAgreement" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-7-differences-between-the-companys-effective-tax-rate-and-the-statutory-tax-rate-details-parentheticals" xlink:href="bcrx-20151231.xsd#statement-note-7-differences-between-the-companys-effective-tax-rate-and-the-statutory-tax-rate-details-parentheticals" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-7-differences-between-the-companys-effective-tax-rate-and-the-statutory-tax-rate-details-parentheticals" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:label="us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-4-minimum-payments-under-operating-lease-obligations-details" xlink:href="bcrx-20151231.xsd#statement-note-4-minimum-payments-under-operating-lease-obligations-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-4-minimum-payments-under-operating-lease-obligations-details" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDue" xlink:label="us-gaap_OperatingLeasesFutureMinimumPaymentsDue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent" xlink:label="us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears" xlink:label="us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears" xlink:label="us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears" xlink:label="us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears" xlink:label="us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter" xlink:label="us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeasesFutureMinimumPaymentsDueCurrent" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeasesFutureMinimumPaymentsDueInTwoYears" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeasesFutureMinimumPaymentsDueInThreeYears" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFourYears" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeasesFutureMinimumPaymentsDueInFiveYears" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeasesFutureMinimumPaymentsDueThereafter" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OperatingLeasesFutureMinimumPaymentsDue" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-consolidated-statements-of-stockholders-equity" xlink:href="bcrx-20151231.xsd#statement-consolidated-statements-of-stockholders-equity" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-consolidated-statements-of-stockholders-equity" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="us-gaap_AdditionalPaidInCapitalMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommonStockMember" xlink:label="us-gaap_CommonStockMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTax" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="us-gaap_RetainedEarningsMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan" xlink:label="us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="us-gaap_StockholdersEquity" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementEquityComponentsAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_CommonStockMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_AdditionalPaidInCapitalMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_RetainedEarningsMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_EquityComponentDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockholdersEquity" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherComprehensiveIncomeLossNetOfTax" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-1-scheduled-maturity-for-the-companys-investments-details" xlink:href="bcrx-20151231.xsd#statement-note-1-scheduled-maturity-for-the-companys-investments-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-1-scheduled-maturity-for-the-companys-investments-details" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughTwoYearsFairValue" xlink:label="bcrx_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughTwoYearsFairValue" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AvailableForSaleSecuritiesDebtMaturitiesMaturingAfterTwoYears" xlink:label="bcrx_AvailableForSaleSecuritiesDebtMaturitiesMaturingAfterTwoYears" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecurities" xlink:label="us-gaap_AvailableForSaleSecurities" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughTwoYearsFairValue" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_AvailableForSaleSecuritiesDebtMaturitiesMaturingAfterTwoYears" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecurities" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-1-summary-of-revenues-details" xlink:href="bcrx-20151231.xsd#statement-note-1-summary-of-revenues-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-1-summary-of-revenues-details" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CSLMember" xlink:label="bcrx_CSLMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CollaborativeAndOtherResearchAndDevelopmentRevenues" xlink:label="bcrx_CollaborativeAndOtherResearchAndDevelopmentRevenues" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_GreenCrossCorporationMember" xlink:label="bcrx_GreenCrossCorporationMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ShionogiAndCoLtdMember" xlink:label="bcrx_ShionogiAndCoLtdMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_USDepartmentOfHealthAndHumanServicesMember" xlink:label="bcrx_USDepartmentOfHealthAndHumanServicesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CounterpartyNameAxis" xlink:label="us-gaap_CounterpartyNameAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:label="us-gaap_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_Revenues" xlink:label="us-gaap_Revenues" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RoyaltyRevenue" xlink:label="us-gaap_RoyaltyRevenue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueGoodsNet" xlink:label="us-gaap_SalesRevenueGoodsNet" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueServicesNetAbstract" xlink:label="us-gaap_SalesRevenueServicesNetAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_CounterpartyNameAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="bcrx_USDepartmentOfHealthAndHumanServicesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="bcrx_GreenCrossCorporationMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="bcrx_ShionogiAndCoLtdMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="bcrx_CSLMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SalesRevenueGoodsNet" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_RoyaltyRevenue" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SalesRevenueServicesNetAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SalesRevenueServicesNetAbstract" xlink:to="bcrx_CollaborativeAndOtherResearchAndDevelopmentRevenues" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_SalesRevenueServicesNetAbstract" xlink:to="us-gaap_Revenues" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-2-furniture-and-equipment-details" xlink:href="bcrx-20151231.xsd#statement-note-2-furniture-and-equipment-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-2-furniture-and-equipment-details" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_LaboratoryEquipmentMember" xlink:label="bcrx_LaboratoryEquipmentMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_LeasedEquipmentMember" xlink:label="bcrx_LeasedEquipmentMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BuildingMember" xlink:label="us-gaap_BuildingMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConstructionInProgressMember" xlink:label="us-gaap_ConstructionInProgressMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="us-gaap_FurnitureAndFixturesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LeaseholdImprovementsMember" xlink:label="us-gaap_LeaseholdImprovementsMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OfficeEquipmentMember" xlink:label="us-gaap_OfficeEquipmentMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="us-gaap_PropertyPlantAndEquipmentGross" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SoftwareDevelopmentMember" xlink:label="us-gaap_SoftwareDevelopmentMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_FurnitureAndFixturesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_OfficeEquipmentMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_SoftwareDevelopmentMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="bcrx_LaboratoryEquipmentMember" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="bcrx_LeasedEquipmentMember" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_LeaseholdImprovementsMember" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_ConstructionInProgressMember" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:to="us-gaap_BuildingMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:to="us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentGross" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-6-stock-incentive-plan-activities-details" xlink:href="bcrx-20151231.xsd#statement-note-6-stock-incentive-plan-activities-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-6-stock-incentive-plan-activities-details" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodAwardsAvailable" xlink:label="bcrx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodAwardsAvailable" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsForfeituresInPeriodAwardsAvailable" xlink:label="bcrx_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsForfeituresInPeriodAwardsAvailable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOtherShareIncreaseDecrease" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOtherShareIncreaseDecrease" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodAwardsAvailable" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsForfeituresInPeriodAwardsAvailable" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriod" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresAndExpirationsInPeriodWeightedAverageExercisePrice" xlink:type="arc"/>
    <link:definitionArc order="14" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:type="arc"/>
    <link:definitionArc order="15" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOtherShareIncreaseDecrease" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-6-weighted-average-assumptions-for-stock-option-awards-granted-to-employees-and-directors-under-the-incentive-plan-details" xlink:href="bcrx-20151231.xsd#statement-note-6-weighted-average-assumptions-for-stock-option-awards-granted-to-employees-and-directors-under-the-incentive-plan-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-6-weighted-average-assumptions-for-stock-option-awards-granted-to-employees-and-directors-under-the-incentive-plan-details" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-6-changes-in-the-number-and-weightedaverage-grantdate-fair-value-details" xlink:href="bcrx-20151231.xsd#statement-note-6-changes-in-the-number-and-weightedaverage-grantdate-fair-value-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-6-changes-in-the-number-and-weightedaverage-grantdate-fair-value-details" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedWeightedAverageGrantDateFairValue" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedWeightedAverageGrantDateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedNumberOfShares" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedNumberOfShares" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedWeightedAverageGrantDateFairValue" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedWeightedAverageGrantDateFairValue" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-7-differences-between-the-companys-effective-tax-rate-and-the-statutory-tax-rate-details" xlink:href="bcrx-20151231.xsd#statement-note-7-differences-between-the-companys-effective-tax-rate-and-the-statutory-tax-rate-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-7-differences-between-the-companys-effective-tax-rate-and-the-statutory-tax-rate-details" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_EffectiveIncomeTaxRateReconciliationTaxCreditOrphanDrugAmount" xlink:label="bcrx_EffectiveIncomeTaxRateReconciliationTaxCreditOrphanDrugAmount" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_IncomeTaxReconciliationIncomeTaxExpenseBenefitExpirationOfCarryForwards" xlink:label="bcrx_IncomeTaxReconciliationIncomeTaxExpenseBenefitExpirationOfCarryForwards" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="us-gaap_IncomeTaxExpenseBenefit" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:label="us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate" xlink:label="us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate" xlink:label="us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxReconciliationNondeductibleExpense" xlink:label="us-gaap_IncomeTaxReconciliationNondeductibleExpense" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxReconciliationOtherAdjustments" xlink:label="us-gaap_IncomeTaxReconciliationOtherAdjustments" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes" xlink:label="us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxReconciliationTaxCreditsResearch" xlink:label="us-gaap_IncomeTaxReconciliationTaxCreditsResearch" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationIncomeTaxExpenseBenefitAtFederalStatutoryIncomeTaxRate" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationStateAndLocalIncomeTaxes" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationNondeductibleExpense" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationChangeInEnactedTaxRate" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_IncomeTaxReconciliationIncomeTaxExpenseBenefitExpirationOfCarryForwards" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationTaxCreditsResearch" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_EffectiveIncomeTaxRateReconciliationTaxCreditOrphanDrugAmount" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationOtherAdjustments" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxReconciliationChangeInDeferredTaxAssetsValuationAllowance" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncomeTaxExpenseBenefit" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-7-reconciliation-of-beginning-and-ending-amount-of-unrecognized-tax-benefits-details" xlink:href="bcrx-20151231.xsd#statement-note-7-reconciliation-of-beginning-and-ending-amount-of-unrecognized-tax-benefits-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-7-reconciliation-of-beginning-and-ending-amount-of-unrecognized-tax-benefits-details" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UnrecognizedTaxBenefits" xlink:label="us-gaap_UnrecognizedTaxBenefits" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions" xlink:label="us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions" xlink:label="us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_UnrecognizedTaxBenefits" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-consolidated-balance-sheets" xlink:href="bcrx-20151231.xsd#statement-consolidated-balance-sheets" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-consolidated-balance-sheets" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CollaborationReceivablesMember" xlink:label="bcrx_CollaborationReceivablesMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_DeferredCollaborationExpenseAssets" xlink:label="bcrx_DeferredCollaborationExpenseAssets" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_DeferredCollaborativeRevenueMember" xlink:label="bcrx_DeferredCollaborativeRevenueMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_DeferredProductSalesMember" xlink:label="bcrx_DeferredProductSalesMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_LeaseFinancingObligationNetOfCurrent" xlink:label="bcrx_LeaseFinancingObligationNetOfCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis" xlink:label="us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsPayableCurrent" xlink:label="us-gaap_AccountsPayableCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsReceivableNetCurrent" xlink:label="us-gaap_AccountsReceivableNetCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccruedLiabilitiesCurrent" xlink:label="us-gaap_AccruedLiabilitiesCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AdditionalPaidInCapital" xlink:label="us-gaap_AdditionalPaidInCapital" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_Assets" xlink:label="us-gaap_Assets" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AssetsAbstract" xlink:label="us-gaap_AssetsAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AssetsCurrent" xlink:label="us-gaap_AssetsCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecuritiesNoncurrent" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtSecuritiesNoncurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommonStockValue" xlink:label="us-gaap_CommonStockValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredCostsCurrent" xlink:label="us-gaap_DeferredCostsCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRentCreditNoncurrent" xlink:label="us-gaap_DeferredRentCreditNoncurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeAxis" xlink:label="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueArrangementTypeDomain" xlink:label="us-gaap_DeferredRevenueArrangementTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueCurrent" xlink:label="us-gaap_DeferredRevenueCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredRevenueNoncurrent" xlink:label="us-gaap_DeferredRevenueNoncurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InterestPayableCurrent" xlink:label="us-gaap_InterestPayableCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InventoryNet" xlink:label="us-gaap_InventoryNet" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LiabilitiesCurrent" xlink:label="us-gaap_LiabilitiesCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NotesPayableCurrent" xlink:label="us-gaap_NotesPayableCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherAssetsNoncurrent" xlink:label="us-gaap_OtherAssetsNoncurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PreferredStockValue" xlink:label="us-gaap_PreferredStockValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="us-gaap_PropertyPlantAndEquipmentNet" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReceivableTypeDomain" xlink:label="us-gaap_ReceivableTypeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReceivableTypeDomain" xlink:label="us-gaap_ReceivableTypeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="us-gaap_StockholdersEquity" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="us-gaap_StockholdersEquityAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TradeAccountsReceivableMember" xlink:label="us-gaap_TradeAccountsReceivableMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ReceivableTypeDomain" xlink:to="bcrx_CollaborationReceivablesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis" xlink:to="us-gaap_ReceivableTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ReceivableTypeDomain" xlink:to="us-gaap_TradeAccountsReceivableMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:to="bcrx_DeferredCollaborativeRevenueMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:to="us-gaap_DeferredRevenueArrangementTypeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:to="bcrx_DeferredProductSalesMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AccountsNotesLoansAndFinancingReceivableByReceivableTypeAxis" xlink:to="us-gaap_ReceivableTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_DeferredRevenueArrangementTypeAxis" xlink:to="us-gaap_DeferredRevenueArrangementTypeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AssetsAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_AvailableForSaleSecuritiesDebtSecuritiesCurrent" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_AccountsReceivableNetCurrent" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_InventoryNet" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_DeferredCostsCurrent" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_AssetsCurrent" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_AvailableForSaleSecuritiesDebtSecuritiesNoncurrent" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_PropertyPlantAndEquipmentNet" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="bcrx_DeferredCollaborationExpenseAssets" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_OtherAssetsNoncurrent" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AssetsAbstract" xlink:to="us-gaap_Assets" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_AccountsPayableCurrent" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_AccruedLiabilitiesCurrent" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_InterestPayableCurrent" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_DeferredRevenueCurrent" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_NotesPayableCurrent" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_LiabilitiesCurrent" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_DeferredRevenueNoncurrent" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="us-gaap_DeferredRentCreditNoncurrent" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:to="bcrx_LeaseFinancingObligationNetOfCurrent" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StockholdersEquityAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_PreferredStockValue" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_CommonStockValue" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_AdditionalPaidInCapital" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_RetainedEarningsAccumulatedDeficit" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_StockholdersEquity" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_LiabilitiesAndStockholdersEquity" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-consolidated-statements-of-comprehensive-loss" xlink:href="bcrx-20151231.xsd#statement-consolidated-statements-of-comprehensive-loss" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-consolidated-statements-of-comprehensive-loss" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CollaborativeAndOtherResearchAndDevelopment" xlink:label="bcrx_CollaborativeAndOtherResearchAndDevelopment" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ComprehensiveIncomeNetOfTax" xlink:label="us-gaap_ComprehensiveIncomeNetOfTax" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CostOfGoodsSold" xlink:label="us-gaap_CostOfGoodsSold" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CostsAndExpenses" xlink:label="us-gaap_CostsAndExpenses" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CostsAndExpensesAbstract" xlink:label="us-gaap_CostsAndExpensesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EarningsPerShareBasicAndDiluted" xlink:label="us-gaap_EarningsPerShareBasicAndDiluted" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments" xlink:label="us-gaap_GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InterestAndOtherIncome" xlink:label="us-gaap_InterestAndOtherIncome" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InterestExpense" xlink:label="us-gaap_InterestExpense" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OperatingIncomeLoss" xlink:label="us-gaap_OperatingIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:label="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="us-gaap_ResearchAndDevelopmentExpense" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RevenuesAbstract" xlink:label="us-gaap_RevenuesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RoyaltyExpense" xlink:label="us-gaap_RoyaltyExpense" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RoyaltyRevenue" xlink:label="us-gaap_RoyaltyRevenue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueGoodsNet" xlink:label="us-gaap_SalesRevenueGoodsNet" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueNet" xlink:label="us-gaap_SalesRevenueNet" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SellingGeneralAndAdministrativeExpense" xlink:label="us-gaap_SellingGeneralAndAdministrativeExpense" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_RevenuesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenuesAbstract" xlink:to="us-gaap_SalesRevenueGoodsNet" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenuesAbstract" xlink:to="us-gaap_RoyaltyRevenue" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenuesAbstract" xlink:to="bcrx_CollaborativeAndOtherResearchAndDevelopment" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_RevenuesAbstract" xlink:to="us-gaap_SalesRevenueNet" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_CostsAndExpensesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_CostOfGoodsSold" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_ResearchAndDevelopmentExpense" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_SellingGeneralAndAdministrativeExpense" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_RoyaltyExpense" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_CostsAndExpenses" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_OperatingIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_InterestAndOtherIncome" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_InterestExpense" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_GainLossOnForeignCurrencyDerivativeInstrumentsNotDesignatedAsHedgingInstruments" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_NetIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_EarningsPerShareBasicAndDiluted" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:type="arc"/>
    <link:definitionArc order="12" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:type="arc"/>
    <link:definitionArc order="13" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_CostsAndExpensesAbstract" xlink:to="us-gaap_ComprehensiveIncomeNetOfTax" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-consolidated-statements-of-cash-flows" xlink:href="bcrx-20151231.xsd#statement-consolidated-statements-of-cash-flows" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-consolidated-statements-of-cash-flows" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_DeferredCollaborationExpenseChangesInOperatingAssetsAndLiabilities" xlink:label="bcrx_DeferredCollaborationExpenseChangesInOperatingAssetsAndLiabilities" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ReceiptPaymentOfForeignCurrencyDerivativeCollateral" xlink:label="bcrx_ReceiptPaymentOfForeignCurrencyDerivativeCollateral" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AmortizationOfFinancingCosts" xlink:label="us-gaap_AmortizationOfFinancingCosts" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease" xlink:label="us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="us-gaap_DepreciationDepletionAndAmortization" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GainLossOnDispositionOfAssets1" xlink:label="us-gaap_GainLossOnDispositionOfAssets1" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncreaseDecreaseInAccountsPayable" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncreaseDecreaseInDeferredRevenue" xlink:label="us-gaap_IncreaseDecreaseInDeferredRevenue" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncreaseDecreaseInDerivativeAssets" xlink:label="us-gaap_IncreaseDecreaseInDerivativeAssets" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncreaseDecreaseInInventories" xlink:label="us-gaap_IncreaseDecreaseInInventories" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xlink:label="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncreaseDecreaseInReceivables" xlink:label="us-gaap_IncreaseDecreaseInReceivables" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncreaseDecreaseInRestrictedCash" xlink:label="us-gaap_IncreaseDecreaseInRestrictedCash" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLossAttributableToReportingEntityAbstract" xlink:label="us-gaap_NetIncomeLossAttributableToReportingEntityAbstract" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt" xlink:label="us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsToAcquireProductiveAssets" xlink:label="us-gaap_PaymentsToAcquireProductiveAssets" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities" xlink:label="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProceedsFromSaleOfFurnitureAndFixtures" xlink:label="us-gaap_ProceedsFromSaleOfFurnitureAndFixtures" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProceedsFromStockOptionsExercised" xlink:label="us-gaap_ProceedsFromStockOptionsExercised" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ProceedsFromStockPlans" xlink:label="us-gaap_ProceedsFromStockPlans" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensation" xlink:label="us-gaap_ShareBasedCompensation" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetIncomeLossAttributableToReportingEntityAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetIncomeLossAttributableToReportingEntityAbstract" xlink:to="us-gaap_NetIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_DepreciationDepletionAndAmortization" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_GainLossOnDispositionOfAssets1" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_ShareBasedCompensation" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_AmortizationOfFinancingCosts" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:to="us-gaap_IncreaseDecreaseInDerivativeAssets" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInReceivables" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInInventories" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="bcrx_DeferredCollaborationExpenseChangesInOperatingAssetsAndLiabilities" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInAccountsPayable" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInDeferredRevenue" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireProductiveAssets" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromSaleOfFurnitureAndFixtures" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_IncreaseDecreaseInRestrictedCash" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_PaymentsToAcquireAvailableForSaleSecuritiesDebt" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromStockOptionsExercised" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_ProceedsFromStockPlans" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="bcrx_ReceiptPaymentOfForeignCurrencyDerivativeCollateral" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:to="us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-1-fair-value-of-the-companys-investments-by-type-details" xlink:href="bcrx-20151231.xsd#statement-note-1-fair-value-of-the-companys-investments-by-type-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-1-fair-value-of-the-companys-investments-by-type-details" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AvailableForSaleSecuritiesAccruedInterest" xlink:label="bcrx_AvailableForSaleSecuritiesAccruedInterest" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecurities" xlink:label="us-gaap_AvailableForSaleSecurities" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:label="us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:label="us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesAmortizedCost" xlink:label="us-gaap_AvailableForSaleSecuritiesAmortizedCost" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CertificatesOfDepositMember" xlink:label="us-gaap_CertificatesOfDepositMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommercialPaperMember" xlink:label="us-gaap_CommercialPaperMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="us-gaap_CorporateDebtSecuritiesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InvestmentTypeAxis" xlink:label="us-gaap_InvestmentTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InvestmentTypeCategorizationMember" xlink:label="us-gaap_InvestmentTypeCategorizationMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InvestmentTypeCategorizationMember" xlink:label="us-gaap_InvestmentTypeCategorizationMember-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_USGovernmentAgenciesDebtSecuritiesMember" xlink:label="us-gaap_USGovernmentAgenciesDebtSecuritiesMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_InvestmentTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InvestmentTypeCategorizationMember" xlink:to="us-gaap_USGovernmentAgenciesDebtSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_InvestmentTypeAxis" xlink:to="us-gaap_InvestmentTypeCategorizationMember-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InvestmentTypeCategorizationMember" xlink:to="us-gaap_CorporateDebtSecuritiesMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InvestmentTypeCategorizationMember" xlink:to="us-gaap_CertificatesOfDepositMember" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_InvestmentTypeCategorizationMember" xlink:to="us-gaap_CommercialPaperMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_InvestmentTypeAxis" xlink:to="us-gaap_InvestmentTypeCategorizationMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesAmortizedCost" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_AvailableForSaleSecuritiesAccruedInterest" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AvailableForSaleSecurities" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-1-summary-of-receivables-details" xlink:href="bcrx-20151231.xsd#statement-note-1-summary-of-receivables-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-1-summary-of-receivables-details" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_CSLLimitedMember" xlink:label="bcrx_CSLLimitedMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ShionogiAndCoLtdMember" xlink:label="bcrx_ShionogiAndCoLtdMember" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_USDepartmentOfHealthAndHumanServicesMember" xlink:label="bcrx_USDepartmentOfHealthAndHumanServicesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis" xlink:label="us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:label="us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BilledRevenuesMember" xlink:label="us-gaap_BilledRevenuesMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:label="us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:label="us-gaap_LegalEntityTypeOfCounterpartyDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReceivablesBillingStatusDomain" xlink:label="us-gaap_ReceivablesBillingStatusDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReceivablesBillingStatusDomain" xlink:label="us-gaap_ReceivablesBillingStatusDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReceivablesNetCurrent" xlink:label="us-gaap_ReceivablesNetCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UnbilledRevenuesMember" xlink:label="us-gaap_UnbilledRevenuesMember" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:to="bcrx_USDepartmentOfHealthAndHumanServicesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:to="us-gaap_LegalEntityTypeOfCounterpartyDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ReceivablesBillingStatusDomain" xlink:to="us-gaap_BilledRevenuesMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis" xlink:to="us-gaap_ReceivablesBillingStatusDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ReceivablesBillingStatusDomain" xlink:to="us-gaap_UnbilledRevenuesMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:to="bcrx_ShionogiAndCoLtdMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:to="bcrx_CSLLimitedMember" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AccountsNotesLoansAndFinancingReceivablesByBillingStatusTypeAxis" xlink:to="us-gaap_ReceivablesBillingStatusDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_AccountsNotesLoansAndFinancingReceivablesByLegalEntityOfCounterpartyTypeAxis" xlink:to="us-gaap_LegalEntityTypeOfCounterpartyDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ReceivablesNetCurrent" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-1-inventory-details" xlink:href="bcrx-20151231.xsd#statement-note-1-inventory-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-1-inventory-details" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InventoryFinishedGoodsNetOfReserves" xlink:label="us-gaap_InventoryFinishedGoodsNetOfReserves" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InventoryNet" xlink:label="us-gaap_InventoryNet" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InventoryWorkInProcessNetOfReserves" xlink:label="us-gaap_InventoryWorkInProcessNetOfReserves" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InventoryWorkInProcessNetOfReserves" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InventoryFinishedGoodsNetOfReserves" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_InventoryNet" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-1-accrued-expenses-details" xlink:href="bcrx-20151231.xsd#statement-note-1-accrued-expenses-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-1-accrued-expenses-details" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AccruedDevelopmentCosts" xlink:label="bcrx_AccruedDevelopmentCosts" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_AccruedInventoryExpense" xlink:label="bcrx_AccruedInventoryExpense" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_DevelopmentCostMember" xlink:label="bcrx_DevelopmentCostMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccruedLiabilitiesCurrent" xlink:label="us-gaap_AccruedLiabilitiesCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccruedProfessionalFeesCurrent" xlink:label="us-gaap_AccruedProfessionalFeesCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent" xlink:label="us-gaap_EmployeeRelatedLiabilitiesCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByLiabilityClassAxis" xlink:label="us-gaap_FairValueByLiabilityClassAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:label="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherAccruedLiabilitiesCurrent" xlink:label="us-gaap_OtherAccruedLiabilitiesCurrent" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TaxesPayableCurrent" xlink:label="us-gaap_TaxesPayableCurrent" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_FairValueByLiabilityClassAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:to="bcrx_DevelopmentCostMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_FairValueByLiabilityClassAxis" xlink:to="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_FairValueByLiabilityClassAxis" xlink:to="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationByLiabilityClassDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EmployeeRelatedLiabilitiesCurrent" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_AccruedDevelopmentCosts" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="bcrx_AccruedInventoryExpense" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AccruedProfessionalFeesCurrent" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_TaxesPayableCurrent" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_OtherAccruedLiabilitiesCurrent" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AccruedLiabilitiesCurrent" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-6-stockbased-compensation-allocation-details" xlink:href="bcrx-20151231.xsd#statement-note-6-stockbased-compensation-allocation-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-6-stockbased-compensation-allocation-details" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="us-gaap_GeneralAndAdministrativeExpenseMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="us-gaap_IncomeStatementLocationAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="us-gaap_IncomeStatementLocationDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="us-gaap_ResearchAndDevelopmentExpenseMember" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_IncomeStatementLocationAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="us-gaap_ResearchAndDevelopmentExpenseMember" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="us-gaap_GeneralAndAdministrativeExpenseMember" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_IncomeStatementLocationAxis" xlink:to="us-gaap_IncomeStatementLocationDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_AllocatedShareBasedCompensationExpense" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-7-components-of-deferred-tax-assets-and-liabilities-details" xlink:href="bcrx-20151231.xsd#statement-note-7-components-of-deferred-tax-assets-and-liabilities-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-7-components-of-deferred-tax-assets-and-liabilities-details" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredIncomeTaxLiabilities" xlink:label="us-gaap_DeferredIncomeTaxLiabilities" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxAssetsDeferredIncome" xlink:label="us-gaap_DeferredTaxAssetsDeferredIncome" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxAssetsGross" xlink:label="us-gaap_DeferredTaxAssetsGross" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxAssetsLiabilitiesNet" xlink:label="us-gaap_DeferredTaxAssetsLiabilitiesNet" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic" xlink:label="us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxAssetsOther" xlink:label="us-gaap_DeferredTaxAssetsOther" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment" xlink:label="us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch" xlink:label="us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost" xlink:label="us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxAssetsValuationAllowance" xlink:label="us-gaap_DeferredTaxAssetsValuationAllowance" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxLiabilitiesUnrealizedCurrencyTransactionGains" xlink:label="us-gaap_DeferredTaxLiabilitiesUnrealizedCurrencyTransactionGains" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsOperatingLossCarryforwardsDomestic" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsResearch" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsDeferredIncome" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost" xlink:type="arc"/>
    <link:definitionArc order="6" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsOther" xlink:type="arc"/>
    <link:definitionArc order="7" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsGross" xlink:type="arc"/>
    <link:definitionArc order="8" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxLiabilitiesUnrealizedCurrencyTransactionGains" xlink:type="arc"/>
    <link:definitionArc order="9" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredIncomeTaxLiabilities" xlink:type="arc"/>
    <link:definitionArc order="10" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsValuationAllowance" xlink:type="arc"/>
    <link:definitionArc order="11" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_DeferredTaxAssetsLiabilitiesNet" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-10-quarterly-financial-information-unaudited-details" xlink:href="bcrx-20151231.xsd#statement-note-10-quarterly-financial-information-unaudited-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-10-quarterly-financial-information-unaudited-details" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EarningsPerShareBasic" xlink:label="us-gaap_EarningsPerShareBasic" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EarningsPerShareBasicAndDiluted" xlink:label="us-gaap_EarningsPerShareBasicAndDiluted" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="us-gaap_EarningsPerShareDiluted" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueNet" xlink:label="us-gaap_SalesRevenueNet" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SalesRevenueNet" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_NetIncomeLoss" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EarningsPerShareBasic" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EarningsPerShareDiluted" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_EarningsPerShareBasicAndDiluted" xlink:type="arc"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.biocryst.com/20151231/role/statement-note-6-number-of-stock-option-awards-exercisable-and-their-weighted-average-exercise-price-details" xlink:href="bcrx-20151231.xsd#statement-note-6-number-of-stock-option-awards-exercisable-and-their-weighted-average-exercise-price-details" xlink:type="simple"/>
  <link:definitionLink xlink:role="http://www.biocryst.com/20151231/role/statement-note-6-number-of-stock-option-awards-exercisable-and-their-weighted-average-exercise-price-details" xlink:type="extended">
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ExercisePriceRange01Member" xlink:label="bcrx_ExercisePriceRange01Member" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ExercisePriceRange02Member" xlink:label="bcrx_ExercisePriceRange02Member" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ExercisePriceRange03Member" xlink:label="bcrx_ExercisePriceRange03Member" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ExercisePriceRange04Member" xlink:label="bcrx_ExercisePriceRange04Member" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ExercisePriceRange05Member" xlink:label="bcrx_ExercisePriceRange05Member" xlink:type="locator"/>
    <link:loc xlink:href="bcrx-20151231.xsd#bcrx_ExercisePriceRange06Member" xlink:label="bcrx_ExercisePriceRange06Member" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain-default" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit" xlink:label="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1" xlink:label="us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2" xlink:label="us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementLineItems" xlink:label="us-gaap_StatementLineItems" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="us-gaap_StatementScenarioAxis" xlink:type="locator"/>
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementTable" xlink:label="us-gaap_StatementTable" xlink:type="locator"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:to="bcrx_ExercisePriceRange01Member" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:to="bcrx_ExercisePriceRange02Member" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:to="bcrx_ExercisePriceRange03Member" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:to="bcrx_ExercisePriceRange04Member" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:to="bcrx_ExercisePriceRange05Member" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:to="bcrx_ExercisePriceRange06Member" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="us-gaap_StatementTable" xlink:to="us-gaap_StatementScenarioAxis" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain-default" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_StatementScenarioAxis" xlink:to="us-gaap_ScenarioUnspecifiedDomain" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain" xlink:type="arc"/>
    <link:definitionArc order="0" xbrldt:closed="true" xbrldt:contextElement="segment" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_StatementTable" xlink:type="arc"/>
    <link:definitionArc order="1" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit" xlink:type="arc"/>
    <link:definitionArc order="2" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions" xlink:type="arc"/>
    <link:definitionArc order="3" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2" xlink:type="arc"/>
    <link:definitionArc order="4" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1" xlink:type="arc"/>
    <link:definitionArc order="5" xbrldt:closed="true" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="us-gaap_StatementLineItems" xlink:to="us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions" xlink:type="arc"/>
  </link:definitionLink>
</link:linkbase>
