Exhibit 99.1

         P R E S S                                               SIS Bancorp
    R E L E A S E



                              FOR IMMEDIATE RELEASE

Date: December 18, 1997                   Contact:  Ting Chang, Vice President
                                                            Investor Relations
                                                                (413) 748-8271

            SIS BANCORP COMPLETES MERGER OF GLASTONBURY BANK & TRUST

Springfield,   Massachusetts,  December  18,  1997--SIS  Bancorp,  Inc.  ("SIS")
(NASDAQ:  SISB) and  Glastonbury  Bank & Trust Company  ("GBT")  (NASDAQ:  GLBT)
announced today that,  having received all necessary  regulatory and stockholder
approvals, the merger of GBT with SIS has been completed. The merger resulted in
GBT becoming a wholly owned banking subsidiary of SIS.

"We are pleased to have been able to  complete  this  transaction  as planned by
year end," said F. William Marshall,  Jr., president and chief executive officer
of SIS Bancorp.  "I am grateful for the efforts of the Board of  Directors,  the
shareholders,  and the employees of SIS and GBT that have made this possible. As
we embark upon a new  beginning,  we will continue to focus on those  activities
that have  brought us success to date.  We look  forward to the new year and the
opportunity to realize the potential of our combined companies," said Marshall.

As a result of the merger,  SIS is now a financial  institution of approximately
$1.7  billion  in total  assets.  SIS is the  holding  company  for  Springfield
Institution   for  Savings,   headquartered   in   Springfield,   MA,  and  GBT,
headquartered  in  Glastonbury,  CT.  These  subsidiaries  operate a total of 34
banking offices in central Connecticut and western Massachusetts.

The merger was completed by the exchange of approximately  1.4 million shares of
SIS common stock for all  outstanding  GBT common  stock.  Based on a twenty-day
average  closing bid price of SIS common stock  through  December  10, 1997,  of
$34.63,   GBT  shareholders   would  be  entitled  to  receive  stock  equal  to
approximately  $25.63  for each share of GBT  common  stock and the  transaction
would be valued at approximately $46.9 million.

After the issuance of the additional shares of common stock for the merger,  SIS
Bancorp will have  approximately  6.9 million shares  outstanding.  These shares
will  continue  to be traded on the  NASDAQ  National  Market  System  under the
current symbol of SISB.

Any  questions  pertaining  to this  release may be forwarded to Ms. Ting Chang,
vice president Investor Relations, at (413) 748-8271.