Exhibit I Joint Filing Agreement pursuant to Rule 13d-1(k)(1)(iii) under the Securities and Exchange Act of 1934, as amended. Each of the undersigned hereby agrees to be included in the filing of the Schedule 13D dated March 13, 2003 with respect to...

                                                                       Exhibit I

Joint Filing Agreement pursuant to Rule 13d-1(k)(1)(iii) under the Securities
and Exchange Act of 1934, as amended.

     Each of the undersigned hereby agrees to be included in the filing of the
Schedule 13D dated March 13, 2003 with respect to the issued and outstanding
Common Stock of Lexicon Genetics Incorporated beneficially owned by each of the
undersigned, respectively.

Dated: March 13, 2003

                                   ESTATE OF XXXXXX X. XXXX


                                         By:  /S/ XXXX X. XXXX
                                            ------------------------------------
                                              Xxxx X. Xxxx
                                              Co-Executor

                                         By:  /S/ XXXXX X. XXXXXX
                                            ------------------------------------
                                              Xxxxx X. Xxxxxx
                                              Co-Executor

                                  /S/ XXXX X. XXXX
                                  ----------------------------------------------
                                  Xxxx X. Xxxx

                                  /S/ XXXXX X. XXXXXX
                                  ----------------------------------------------
                                  Xxxxx X. Xxxxxx

                                  /S/ XXXXXXX X. XXXXXX
                                  ----------------------------------------------
                                  Xxxxxxx X. XxXxxx