(Exhibit 24.02)


                          AMBAC FINANCIAL GROUP, INC.

                               Power of Attorney
                               -----------------



               KNOW ALL MEN BY THESE PRESENTS that the undersigned director of
     Ambac Financial Group, Inc., a Delaware corporation, hereby constitutes and
     appoints each of Frank J. Bivona  and Anne G. Gill, as his true and lawful
     attorney-in-fact and agent, with full power of substitution, for him and in
     his name, place and stead, in any and all capacities, to sign the Annual
     Report on Form 10-K for the year ended December 31, 2000, to be filed with
     the Securities and Exchange Commission and the New York Stock Exchange, and
     any and all amendments thereto, and any and all instruments and documents
     filed as a part of or in connection with the said Form 10-K or amendments
     thereto, and does hereby grant unto each said attorney-in-fact and agent
     full power and authority to do and perform each and every act and thing
     requisite and necessary to be done in and about the premises, as fully to
     all intents and purposes as he or she might or could do in person, hereby
     ratifying and confirming all that each said attorney-in-fact and agent
     shall do or cause to be done by virtue hereof.

               IN WITNESS WHEREOF, the undersigned has subscribed these presents
     as of this 23/rd/ day of March, 2001.



                                         /s/ Michael A. Callen
                                         ---------------------
                                         Michael A. Callen