Exhibit 5.1

                       [LETTERHEAD OF FIRSTAR CORPORATION]




                                 October 7, 1999


Board of Directors
Firstar Corporation
Firstar Center
777 East Wisconsin Avenue
Milwaukee, WI  53202

         Re:      Post-Effective Amendment No. 1 on
                  Form S-8 Registration Statement

Ladies and Gentlemen:

         Reference  is  made  to  the  Post-Effective  Amendment  No.  1 to  the
Registration  Statement  on  Form  S-8  of  Firstar  Corporation,   a  Wisconsin
corporation  ("Firstar")  relating to certain  stock-based  employee or director
benefit plans (collectively,  the "Plans") of Mercantile  Bancorporation Inc., a
Missouri corporation ("Mercantile") concurrently being filed with the Securities
and  Exchange  Commission  (the  "Registration  Statement").   Pursuant  to  the
Registration Statement,  Firstar's common stock, $ 0.01 par value per share (the
"Common Stock") and related Preferred Share Purchase Rights (the "Rights"), will
be issued,  or reserved for issuance,  under the Plans pursuant to the merger of
Mercantile with and into Firstar (the "Merger") under the terms of the Agreement
and Plan of Merger,  dated as of April 30, 1999, as amended as of June 17, 1999,
by and between Mercantile and Firstar (the "Agreement"). The terms of the Rights
are as set forth in the Rights Agreement, dated as of November 23, 1998, between
Firstar and Firstar Bank Milwaukee N.A., as rights agent thereunder.

     For  purposes  of  this  opinion,  I have  examined,  among  other  things,
originals or copies,  certified or otherwise  identified to my satisfaction,  of
such  documents  as I have deemed  necessary or  appropriate  as a basis for the
opinions set forth herein.

         On the basis of the foregoing, it is my opinion that:

1.            The  shares  of  Common   Stock   offered  as  set  forth  in  the
              Registration Statement and relevant Plan documents, when issued in
              accordance  with  their  respective  terms  and the  terms  of the
              Agreement and the respective Plans, will be legally issued,  fully
              paid   and   nonassessable,   except   that,   with   respect   to
              nonassessability,  Section  622(2)(b)  of the  Wisconsin  Business
              Corporation  Law, and  judicial  interpretations  thereof,  impose
              liability  upon  shareholders  for unpaid wage claims of Firstar's
              employees, not exceeding six months service in any one case.


2.            The  Rights  to be  issued  with the  Common  Stock,  when  issued
              pursuant  to the terms of the  Rights  Agreement,  will be validly
              issued.

         I am a member of the bar of the State of  Wisconsin.  This  opinion  is
limited to the federal laws of the United  States of America and the laws of the
State of Wisconsin.

         I hereby  consent  to the  filing of this  opinion as an exhibit to the
Registration  Statement  and to the use of my name  whenever  it appears in such
Registration Statement,  including the applicable Prospectus constituting a part
thereof, as originally filed or as subsequently amended.

                                                       Very truly yours,


                                                       /s/  EMILY CANEDO, ESQ.
                                                       ------------------------
                                                       Name: Emily Canedo, Esq.
                                                       Title: Corporate Attorney