
<PAGE>

                                                                     Exhibit 5.1



Morgan, Lewis & Bockius LLP
2000 One Logan Square
Philadelphia, PA  19103



October 8, 1997 

Infonautics, Inc.
900 West Valley Road, Suite 1000
Wayne, Pennsylvania  19087

Re: Infonautics, Inc. - Form S-8 Registration Statement 
    Relating to 1996 Equity Compensation Plan

Ladies and Gentlemen:

As your counsel, we have assisted in the preparation of the above-referenced
registration statement (the "Registration Statement") for filing with the
Securities and Exchange Commission pursuant to the Securities Act of 1933, as
amended (the "Act"), and the rules and regulations promulgated thereunder.

The Registration Statement relates to 500,000 shares of Class A Common Stock, 
no par value (the "Class A Common Stock"), of Infonautics, Inc. (the 
"Registrant"), which may be issued pursuant to the 1996 Equity Compensation 
Plan, as amended and restated as of April 1, 1997 and as of September 23, 1997
(the "Plan").  We have examined the Registrant's Articles of Incorporation,
as amended, By-Laws, as amended, minutes and such other documents, and have
made such inquiries of the Registrant's officers, as we have deemed
appropriate.  In our examination, we have assumed the genuineness of all
signatures, the authenticity of all items submitted to us as originals, and the
conformity with originals of all items submitted to us as copies.

Based upon the foregoing, it is our opinion that the Registrant's Class A Common
Stock originally issued by the Registrant to eligible participants through the
Plan, when issued and delivered as contemplated by the Plan, will be legally
issued, fully paid and non-assessable.

We hereby consent to the use of this opinion as Exhibit 5.1 to the Registration
Statement.  In giving such consent, we do not thereby admit that we are acting
within the category of persons whose consent is required under Section 7 of the
Act or the rules or regulations of the Securities and Exchange Commission
thereunder.

Very truly yours,


/s/Morgan, Lewis & Bockius LLP

