EXHIBIT 99.1


                              FOR IMMEDIATE RELEASE

Date: July 20, 1998                                                     Contact:
                                                      Ting Chang, vice president
                                                              Investor Relations
                                                                  (413) 748-8271


                     PEOPLES HERITAGE TO ACQUIRE SIS BANCORP
          Extends Franchise into Western Massachusetts and Connecticut


Portland, Maine, July 20, 1998 - Peoples Heritage Financial Group, Inc. (NASDAQ:
PHBK), a multi-state bank and financial  services holding company  headquartered
in Portland,  Maine, has reached a definitive  agreement to acquire Springfield,
Massachusetts-based  SIS Bancorp,  Inc.  (NASDAQ:  SISB).  The acquisition  will
create a $12 billion banking company,  materially enhance Peoples' Massachusetts
banking franchise and extend it into the western part of the state. Peoples will
also gain its initial presence in Connecticut through SIS's Glastonbury Bank and
Trust Company.

Under the terms of the merger  agreement,  shareholders of SIS will receive 2.25
shares of Peoples  Heritage  commons  stock for each  whole  share of SIS common
stock plus cash in lieu of any  fractional  share.  Approximately  16.8  million
shares of  Peoples  Heritage  common  stock  will be issued in the  transaction,
including  shares  issuable upon the exercise of substitute  stock options.  The
exchange will be tax free and accounted for as a pooling of interests.

Based on a closing price of Peoples  Heritage common stock of $25 7/16 per share
on July 17, 1998, the deal is valued at $427.7 million and SIS shareholders will
receive  $57.23 of Peoples  Heritage  common  stock for each share of SIS common
stock.  The price  equates to 3.13 times the book value of SIS at March 31, 1998
and 21.4 times SIS' estimated 1999 earnings.

"We continue to grow as planned,  contiguous to our existing  franchise  through
the  acquisition  of  another  strong  community  bank,"  said  William J. Ryan,
Chairman, President and Chief Executive Officer of Peoples Heritage.


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His  comments  were echoed by F.  William  Marshall,  Jr.,  President  and Chief
Executive  Officer of SIS: "An essential  factor in our decision to move forward
with this opportunity is Peoples  Heritage's  strong commitment to serving local
communities."  Mr.  Marshall  will  continue  to serve as  President  and  Chief
Executive  Officer  of SIS and  will be named  Vice  Chairman  of the  Executive
Management Committee of Peoples Heritage Financial Group.

Under the terms of the  agreement,  SIS Bank will  continue to operate under its
own name.  Glastonbury  Bank & Trust  Company,  recently  acquired by SIS,  will
continue to operate as a Connecticut bank.

The agreement is subject to the receipt of approval of the  shareholders  of SIS
and requisite regulatory approvals.  It is anticipated that the transaction will
be  completed  by the end of 1998.  As part of the  agreement,  SIS gave Peoples
Heritage an option to purchase  19.9  percent of its  outstanding  common  stock
under certain circumstances.

Peoples  Heritage has $9.8  billion in assets and operates 194 branches  through
its three  banking  subsidiaries:  Peoples  Heritage  Bank with Maine's  leading
deposit market share,  Bank of New Hampshire with that state's  leading  deposit
market share,  and Family Bank in  Massachusetts.  The  acquisition  of SIS will
increase the number of the Company's  banking offices in Massachusetts to 56 and
create its first banking  presence in  Connecticut  with eight  offices,  mostly
located  in the  central  Connecticut.  SIS has assets of $1.8  billion  and the
leading market share in Hampden County in  Massachusetts.  Peoples Heritage will
have  the  eighth  largest  deposit  market  share  in  Massachusetts  following
completion of the transaction.

A conference  call on the acquisition  announcement  and Peoples' second quarter
earnings, also announced today, will be held at 10 a.m. eastern time.

For PHBK investor  inquiries  please contact Brian  Arsenault,  Vice  President,
Investor Relations and Corporate Communications, at (207) 761-8517.

For SIS investor inquiries, please contact Ting Chang, Vice President,  Investor
Relations and Corporate Planning, at (413)748-8271.

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