<PAGE>
 
                UNITED STATES SECURITIES AND EXCHANGE COMMISSION
                             450 Fifth Street, N.W.
                             Washington, D.C. 20549

                                Amendment No. 1

                                  FORM 10-K/A

[X]  Annual Report Pursuant to Section 13 or 15(d) of the Securities Exchange
     Act of 1934 [FEE REQUIRED] For the Fiscal Year Ended December 31, 1998

                                       OR

[_]  Transition Report Pursuant to Section 13 or 15(d) of the Securities
     Exchange Act of 1934 [NO FEE REQUIRED]

     For the transition period from _______________ to ______________________

                        Commission File Number  0-22641
                                              ------------

                             PEOPLES BANCORP, INC.
                             ---------------------
            (Exact name of registrant as specified in its charter)

             Delaware                                  22-6764023 
   ------------------------------                  ---------------- 
  (State or other jurisdiction of                  (I.R.S. Employer
  incorporation or organization)                 Identification Number)

 134 Franklin Corner Road, Lawrenceville, New Jersey           08648-0950
------------------------------------------------------         ----------
      (Address of Principal Executive Offices)                  Zip Code

                                (609) 844-3100
                        -------------------------------
                        (Registrant's telephone number)

Securities Registered Pursuant to Section 12(b) of the Act:   None
<TABLE> 

<S>                                                          <C> 
Securities Registered Pursuant to Section 12(g) of the Act:  Common Stock, par value $.10 per share
                                                             --------------------------------------
                                                                        (Title of Class)
</TABLE> 

     Indicate by check mark whether the Registrant (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding twelve months (or for such shorter period that the
Registrant was required to file such reports) and (2) has been subject to such
requirements for the past 90 days. YES   X   NO
                                        ---      ---

     Indicate by check mark if disclosure of delinquent filers pursuant to Item
405 of Regulation S-K is not contained herein, and will not be contained, to the
best of Registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of this Form 10-K or any amendment to this
Form 10-K. [_].

     As of February 26, there were issued and outstanding 32,684,198 shares of
the Registrant's Common Stock. The aggregate market value of the voting stock
held by nonaffiliates of the Registrant, computed by reference to the closing
sales price of the Registrant's stock, as reported on the Nasdaq National Market
on February 26, 1999, was approximately $318.7 million.

                       DOCUMENTS INCORPORATED BY REFERENCE

None

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                                   SIGNATURES

         Pursuant to the requirements of Section 13 or 15(d) of the Securities
Exchange Act of 1934, the registrant has duly caused this report to be signed on
its behalf by the undersigned, thereunto duly authorized.

                                      PEOPLES BANCORP, INC.


Date: March 26, 1999                  By:  /s/ Wendell T. Breithaupt     
                                           ------------------------------
                                           Wendell T. Breithaupt
                                           President and Chief Executive Officer
                                           (Principal Executive Officer)

         Pursuant to the requirements of the Securities Exchange of 1934, this
report has been signed below by the following persons on behalf of the
registrant and in the capacities and on the dates indicated.



By:  /s/ Dan A. Chila                      By: /s/ John B. Sill, Jr.   
     --------------------------------          ---------------------------------
     Dan A. Chila, Senior Vice                 John B. Sill, Jr., Chairman
     President and Chief Financial 
     Officer (Principal  Financial
     and Accounting Officer)

Date:  March 26, 1999                          Date:  March 26, 1999     

By:  /s/ Miles W. Truesdell, Jr.           By: /s/ Peter S. Longstreth    
     --------------------------------          ---------------------------------
     Miles W. Truesdell, Jr., Director         Peter S. Longstreth, Director

Date:  March 26, 1999                           Date:  March 26, 1999    

By:  /s/ George A. Pruitt                  By: /s/ George W. Reinhard     
     --------------------------------          ---------------------------------
     George A. Pruitt, Director                George W. Reinhard, Director

Date:  March 26, 1999                          Date:  March 26, 1999    

By:  /s/ Charles E. Stokes                 By: /s/ Raymond E. Trainer     
     --------------------------------          ---------------------------------
     Charles E. Stokes, Director               Raymond E. Trainer, Director

Date:  March 26, 1999                          Date:  March 26, 1999      
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