1
                                                                   EXHIBIT 3.18

                          CERTIFICATE OF INCORPORATION

                                       OF

                        PIONEER VALLEY HEALTH PLAN, INC.


            FIRST:  The name of the Corporation is Pioneer Valley Health Plan,
Inc. (hereinafter the "Corporation").

            SECOND: The address of the registered office of the Corporation in
the State of Delaware is 1209 Orange Street, in the City of Wilmington, County
of New Castle. The name of its registered agent at that address is The
Corporation Trust Company.

            THIRD: The purpose of the Corporation is to engage in any lawful act
or activity for which a corporation may be organized under the General
Corporation Law of the State of Delaware as set forth in Title 8 of the Delaware
Code (the "GCL").

            FOURTH: The total number of shares of stock which the Corporation
shall have authority to issue is 1,000 shares of Common Stock, each having a par
value of one penny ($.01).

            FIFTH:  The name and mailing address of the Sole Incorporator is as
follows:

      Name                          Address
      ----                          -------

Deborah M. Reusch                   P.O. Box 636
                                    Wilmington, DE  19899

            SIXTH: The following provisions are inserted for the management of
the business and the conduct of the affairs of the Corporation, and for further
definition, limitation and regulation of the powers of the Corporation and of
its directors and stockholders:

            (1) The business and affairs of the Corporation shall be managed by
      or under the direction of the Board of Directors.
   2
            (2) The directors shall have concurrent power with the stockholders
      to make, alter, amend, change, add to or repeal the By-Laws of the
      Corporation.

            (3) The number of directors of the Corporation shall be as from time
      to time fixed by, or in the manner provided in, the By-Laws of the
      Corporation. Election of directors need not be by written ballot unless
      the By-Laws so provide.

            (4) A director of the Corporation shall not be personally liable to
      the Corporation or its stockholders for monetary damages for breach of
      fiduciary duty as a director, except as is expressly provided by the GCL,
      as the same exists or may hereafter be amended to further limit or
      eliminate such liability. Any repeal or modification of this Article SIXTH
      by the stockholders of the Corporation shall not adversely affect any
      right or protection of a director of the Corporation existing at the time
      of such repeal or modification with respect to acts or omissions occurring
      prior to such repeal or modification.

            (5) In addition to the powers and authority hereinbefore or by
      statute expressly conferred upon them, the directors are hereby empowered
      to exercise all such powers and do all such acts and things as may be
      exercised or done by the Corporation, subject, nevertheless, to the
      provisions of the GCL, this Certificate of Incorporation, and any By-Laws
      adopted by the stockholders; provided, however, that no By-Laws hereafter
      adopted by the stockholders shall invalidate any prior act of the
      directors which would have been valid if such By-Laws had not been
      adopted.

            SEVENTH: Meetings of stockholders may be held within or without the
State of Delaware, as the By-Laws may provide. The books of the Corporation may
be kept (subject to any provision contained in the GCL) outside the State of
Delaware at such place or places as may be designated from time to time by the
Board of Directors or in the By-Laws of the Corporation.

            EIGHTH: The Corporation reserves the right to amend, alter, change
or repeal any provision contained in this Certificate of Incorporation, in the
manner now or hereafter prescribed by statute, and all rights conferred upon
stockholders herein are granted subject to this reservation.


                                       2
   3
            I, THE UNDERSIGNED, being the Sole Incorporator hereinbefore named,
for the purpose of forming a corporation pursuant to the GCL, do make this
Certificate, hereby declaring and certifying that this is my act and deed and
the facts herein stated are true, and accordingly have hereunto set my hand this
23rd day of September, 1999.


                              /s/ Deborah M. Reusch
                              --------------------------------
                              Deborah M. Reusch
                              Sole Incorporator


                                        3