EXHIBIT 99.1

                                  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, 2002, and to which this Certification is attached as
                  Exhibit 99.1 (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 14th day of August, 2002.

Date:   August 14, 2002               By:  /s/ ARTHUR T. SANDS
                                           -------------------------------------
                                           Arthur T. Sands, M.D., Ph.D.
                                           President and Chief Executive Officer


Date:   August 14, 2002               By:  /s/ JULIA P. GREGORY
                                           -------------------------------------
                                           Julia P. Gregory
                                           Executive Vice President and
                                           Chief Financial Officer