EXHIBIT 99.1

                  CERTIFICATION BY PRINCIPAL EXECUTIVE OFFICER
                       PURSUANT TO 18 U.S.C. SECTION 1350,
                       AS ADOPTED PURSUANT TO SECTION 906
                        OF THE SARBANES-OXLEY ACT OF 2002

     In connection with the accompanying Quarterly Report on Form 10-Q of Ligand
Pharmaceuticals Inc. for the quarter ended March 31, 2003, I, David E. Robinson,
Chairman, President and Chief Executive Officer of Ligand Pharmaceuticals Inc.,
hereby certify pursuant to 18 U.S.C. Section 1350, as adopted pursuant to
Section 906 of the Sarbanes-Oxley Act of 2002, to the best of my knowledge and
belief, that:

     (1) such Quarterly Report on Form 10-Q for the quarter ended March 31,
2003, fully complies with the requirements of section 13(a) or 15(d) of the
Securities Exchange Act of 1934; and

     (2) the information contained in such Quarterly Report on Form 10-Q for the
quarter ended March 31, 2003, fairly presents, in all material respects, the
financial condition and results of operations of Ligand Pharmaceuticals Inc.

Date: May 5, 2003


                             /S/DAVID E. ROBINSON
                             -----------------------------------------
                             David E. Robinson
                             CHAIRMAN, PRESIDENT AND CHIEF EXECUTIVE OFFICER