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                                                                    EXHIBIT 32.1

                                  CERTIFICATION

        Pursuant to Section 906 of the Sarbanes-Oxley Act of 2002 (18 U.S.C.
1350, as adopted), Arthur T. Sands, M.D., Ph.D., Chief Executive Officer of
Lexicon Genetics Incorporated ("Lexicon"), and Julia P. Gregory, Chief Financial
Officer of Lexicon, each hereby certify that:

         1.   Lexicon's Quarterly Report on Form 10-Q for the period ended June
              30, 2003, and to which this Certification is attached as Exhibit
              32.1 (the "Periodic Report"), fully complies with the requirements
              of section 13(a) or section 15(d) of the Securities Exchange Act
              of 1934, and

         2.   The information contained in the Periodic Report fairly presents,
              in all material respects, the financial condition and results of
              operations of Lexicon.

        IN WITNESS WHEREOF, the undersigned have set their hands hereto as of
the 6th day of August, 2003.

Date: August 6, 2003                 By: /s/ ARTHUR T. SANDS
                                         --------------------------------------
                                         Arthur T. Sands, M.D., Ph.D.
                                         President and Chief Executive Officer

Date: August 6, 2003                 By: /s/ JULIA P. GREGORY
                                         --------------------------------------
                                         Julia P. Gregory
                                         Executive Vice President and
                                         Chief Financial Officer

