EXHIBIT 5.1

                       [Vinson & Elkins L.L.P. Letterhead]

October 20, 2006

Lexicon Genetics Incorporated
8800 Technology Forest Place
The Woodlands, Texas 77381

Ladies and Gentlemen:

     We have acted as counsel for Lexicon Genetics Incorporated, a Delaware
corporation (the "Company"), in connection with the preparation of the
prospectus dated June 13, 2006 (the "Prospectus") and the prospectus supplement
dated October 20, 2006 (the "Prospectus Supplement") with respect to the
Registration Statement on Form S-3 (Registration No. 333-122214) (the
"Registration Statement") filed by the Company with the Securities and Exchange
Commission (the "Commission") under the Securities Act of 1933, as amended (the
"Securities Act"), relating to the issuance and sale of up to 10,582,011 shares
of the Company's common stock, par value $.001 per share (the "Shares"). The
Shares are being offered, issued and sold pursuant to a Placement Agency
Agreement dated October 20, 2006 by and among the Company, Banc of America
Securities LLC and Lazard Capital Markets LLC (the "Placement Agency
Agreement").

     In our capacity as your counsel for the matter referred to above, we have
examined such statutes, including the Delaware General Corporation Law,
corporate records, documents, certificates of public officials and other
instruments and documents as we deemed necessary or advisable for purposes of
this opinion. In giving such opinions, we have relied upon certificates of
officers of the Company with respect to the accuracy of the material factual
matters contained in such certificates. In making our examination, we have
assumed the authenticity of all documents submitted to us as originals and the
conformity with the original documents of all documents submitted to us as
copies.

     In connection with this opinion, we have assumed that the Shares will be
issued and sold in the manner set forth in the Prospectus Supplement and the
Placement Agency Agreement.

     On the basis of the foregoing, and subject to the assumptions, limitations
and qualifications set forth herein, we are of the opinion that the Shares, when
issued and delivered against payment therefor as provided in the Placement
Agency Agreement, will be validly issued, fully paid and non-assessable.



     The opinions expressed herein are limited in all respects to the
Constitution of the State of Delaware and the Delaware General Corporation Law,
as interpreted by the courts of the State of Delaware and the United States, and
we are expressing no opinion as to the effect of the laws of any other
jurisdiction.

     We hereby consent to references to this firm under the caption "Legal
Matters" in the Prospectus Supplement and to the filing of this opinion as an
exhibit to a Current Report on Form 8-K of the Company. In giving this consent,
we do not admit that we are within the category of persons whose consent is
required under Section 7 of the Securities Act and the rules and regulations
thereunder.

                                        Very truly yours,


                                        /s/ Vinson & Elkins L.L.P.