SECURITIES AND EXCHANGE COMMISSION
                             WASHINGTON, D.C. 20549


                                    FORM 8-K

                                 CURRENT REPORT
                     Pursuant to Section 13 or 15(d) of the
                         Securities Exchange Act of 1934

                Date of Report (Date of Earliest Event Reported)
                          July 21, 1998 (July 20, 1998)

                                 --------------

                                SIS BANCORP, INC.
               (exact name of registrant as specified in charter)

Massachusetts                        000-20809              04-3303264
(State or Other Jurisdiction         (Commission            (IRS Employer
of Incorporation)                    File Number)           Identification No.)


1441 Main Street
Springfield, Massachusetts                           01102
(address of principal office)                        (Zip Code)


                                 (413) 748-8000
              (Registrant's telephone number, including area code)


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Item 5: Other Events

     On July 20, 1998, SIS Bancorp,  Inc. (the "Company")  announced that it and
Peoples Heritage  Financial Group,  Inc.  ("PHFG") had entered into an Agreement
and Plan of  Merger,  dated as of July 20,  1998 (the  "Agreement"),  which sets
forth the terms and  conditions  pursuant to which the  Company  would be merged
with and into Peoples  Heritage Merger Corp., a wholly-owned  subsidiary of PHFG
(the "Merger").  The Agreement provides, among other things, that as a result of
the Merger,  each  outstanding  share of common stock of the Company (subject to
certain  exceptions)  will be converted into the right to receive 2.25 shares of
PHFG's  common  stock,  plush  cash in lieu of any  fractional  share  interest.
Consummation of the Merger is subject to a number of conditions,  including, but
not  limited  to,  (i) the  approval  of the  Agreement  and the  Merger  by the
shareholders  of the  Company  and  (ii) the  receipt  of  requisite  regulatory
approvals.

     Pursuant  to  the  Agreement,  the  Company's  Massachusetts-based  banking
subsidiary,  Springfield  Institution  for Savings  (d/b/a "SIS Bank"),  will be
merged into PHFG's Massachusetts-based banking subsidiary,  Family Bank, FSB. It
is PHFG's  current  intention  to conduct  business in the market areas in which
Springfield Institution for Savings conducted business prior to such bank merger
under  the  name  "SIS  Bank".   The   Company's   other   banking   subsidiary,
Connecticut-based Glastonbury Bank and Trust Company, will be held as a separate
institution.

     Pursuant  to the  Agreement,  one  director  of the  Company  will become a
director of PHFG and F. William Marshall, Jr., the Company's President and Chief
Executive Officer, will become an executive officer of PHFG and Vice Chairman of
PHFG's Senior Management Committee.

     In connection with the Agreement, PHFG and the Company entered into a Stock
Option  Agreement,  dated as of July 20,  1998,  pursuant  to which the  Company
granted PHFG an option (the "Option") to purchase up to 1,385,383  shares of the
Company's  common stock  (subject to  adjustment  as set forth  therein),  which
represents  19.9% of the  Company's  outstanding  shares of common  stock,  at a
purchase price of $44.00 per share (subject to adjustment as set forth therein).
The Option will become  exercisable  upon the occurrence of certain  events,  as
specified in the Stock Option  Agreement,  none of which has occurred as of July
20, 1998.

     The  press  release  issued by PHFG and the  Company  with  respect  to the
announcement of the transaction  described  herein is attached hereto as Exhibit
99.1 and is hereby incorporated herein by reference in its entirety.

Item 7: Financial Statements,  Pro Forma Financial Information and Exhibits

     (a)      Not applicable.

     (b)      Not applicable.

     (c) The following exhibit is included with this Report:

                      Exhibit 99.1      Press Release dated July 20, 1998















Signatures:  Under the requirements of the Securities  Exchange Act of 1934, the
registrant  has duly  caused  this  report  to be  signed  on its  behalf by the
undersigned, hereunto duly authorized.


                                         SIS BANCORP, INC.

Dated: July 21, 1998                     By: /s/ F. William Marshall, Jr.
                                            -----------------------------
                                            F. William Marshall, Jr.
                                            President & Chief Executive Officer