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 Annual Report on Form 10-K for the year ended December 31,
          2004, 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
11th day of March, 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