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 March
            31, 2005, 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
5th day of May, 2005.

                                   By:  /s/ Arthur T. Sands
                                       --------------------------------------
                                        Arthur T. Sands, M.D., Ph.D.
                                        President and Chief Executive Officer

                                   By:  /s/ Julia P. Gregory
                                       --------------------------------------
                                        Julia P. Gregory
                                        Executive Vice President,
                                        Corporate Development and
                                        Chief Financial Officer