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                                                                     EXHIBIT 5.1





                          [COHEN & GRIGSBY LETTERHEAD]



                                November 9, 1998



Board of Directors of
F.N.B. Corporation
Hermitage Square
Hermitage, PA 16158

Gentlemen:

          We have been asked to render this opinion in connection with the
filing by F.N.B. Corporation, a Pennsylvania corporation (the "Company"), of a
Registration Statement on Form S-4 (the "Registration Statement") with the
Securities and Exchange Commission to register shares of the Company's common
stock, par value $2.00 per share (the "Common Stock"), to be issued to the
shareholders of Guaranty Bank & Trust Company, a Florida state banking
corporation ("Guaranty"), upon consummation of the merger (the "Merger") of
Guaranty with and into Southwest Interim Bank No. 5, N.A., a national banking
association to be chartered under the laws of the United States ("Interim"), in
accordance with the terms of the Agreement and Plan of Merger dated as of August
20, 1998, between the Company and Guaranty and to be joined in by Interim, as
amended by the First Amendment thereto dated October 15, 1998 (the "Agreement").

          We have not represented the Company in connection with the
negotiation, execution or delivery of the Agreement or the Merger. In rendering
this Opinion, we have made no investigation or inquiry other than review of the
Agreement, the draft Registration Statement, the resolutions adopted by the
Executive Committee of the Board of Directors of the Company on June 22, 1998
with respect to the Merger, the Agreement and the transactions related thereto
and the Company's Articles of Incorporation and Bylaws, as amended.

          In rendering this opinion, we have made the following assumptions
regarding factual matters:
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Boards of Directors of
F.N.B. Corporation
November 9, 1998
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          1.     The Agreement is enforceable as written.

          2.     Interim will be duly organized as a wholly-owned subsidiary 
of the Company and will join in and become a party to the Agreement.

          3.     Each document submitted to us for review and each document
obtained by us from any governmental authority is accurate and complete, each
such document that is an original is authentic, each such document that is a
copy conforms to an authentic original and all signatures on each such document
are genuine. All official public records from which any such document, or the
information contained in any such document, was obtained are accurate and
complete and have been properly indexed and filed.

          4.     All consideration to be paid by the shareholders of Guaranty 
who receive shares of Common Stock in the Merger will be paid in accordance 
with the terms of the Agreement.

          Based upon the foregoing examination and assumptions, and subject to 
compliance with applicable federal and state securities and "Blue Sky" laws, in 
our opinion the shares of Common Stock to be issued to the shareholders of 
Guaranty upon consummation of the Merger, when issued in accordance with the 
terms of the Agreement, will be validly issued, fully-paid and non-assessable 
shares of Common Stock of the Company.

          We are members of the Bar of the Commonwealth of Pennsylvania, and our
opinion expressed herein is limited to the laws of the Commonwealth of
Pennsylvania and the United States, in each case as currently in effect, and we
assume no responsibility as to the applicability to the matters covered hereby
of the laws of any other jurisdiction. To the extent that the Agreement is
governed by the laws of a jurisdiction other than the Commonwealth of
Pennsylvania, our opinion herein as it relates to the Agreement is given as if
the laws of the Commonwealth of Pennsylvania govern such agreement; we express
no opinion as to the jurisdiction whose laws actually govern.
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Boards of Directors of
F.N.B. Corporation
November 9, 1998
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           This opinion letter is given solely as of the date hereof and is
limited to the matters expressly set forth herein. We hereby consent to the 
reference to us in the Prospectus of the Company and Proxy Statement of 
Guaranty constituting part of the Registration Statement and to the inclusion 
of this letter as an exhibit to the Registration Statement.


                                        Very truly yours,

                                        /s/ Cohen & Grigsby, P.C.
                                        
                                        COHEN & GRIGSBY, P.C.