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                                                                    Exhibit 4.3
                                                                    ----------- 

                             ASPEN TECHNOLOGY, INC.

                       1995 EMPLOYEES' STOCK PURCHASE PLAN

         1. DEFINITIONS. As used in this 1995 Employees' Stock Purchase Plan of
Aspen Technology, Inc., the following terms shall have the meanings respectively
assigned to them below:

          (a)  BENEFICIARY means the person designated as beneficiary on the
               Optionee's Membership Agreement or, if no such beneficiary is
               named, the person to whom the Option is transferred by will or
               under the applicable laws of descent and distribution.

          (b)  CODE means the Internal Revenue Code of 1986, as amended.

          (c)  COMMITTEE means a committee of the board of directors of the
               Company composed exclusively of disinterested directors.

          (d)  COMPANY means Aspen Technology, Inc., a Massachusetts
               corporation.

          (e)  COMPENSATION means annual compensation, including commissions,
               overtime and bonuses for the most recently completed calendar
               year.

          (f)  ELIGIBLE EMPLOYEE means a person who is eligible under the
               provisions of Section 7 to receive an Option as of a particular
               Grant Date.

          (g)  EXERCISE DATE means a date not more than 27 months after a Grant
               Date, as determined by the Committee, on which Options must, if
               ever, be executed.

          (h)  GRANT DATE means a date specified by the Committee on which
               Options are to be granted to Eligible Employees.

          (j)  MARKET VALUE means, as of a particular date, the value as
               determined by the Committee in accordance with applicable
               provisions of the Code and Treasury Department rulings and
               regulations thereunder or, if applicable, the closing price of
               the Stock reported by NASDAQ in The Wall Street Journal on such
               date.

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          (k)  MEMBERSHIP AGREEMENT means an agreement whereby an Optionee
               authorizes the Company to withhold payroll deductions from his or
               her Compensation.

          (l)  OPTION means an option to purchase shares of Stock granted under
               the Plan.

          (m)  OPTIONEE means an Eligible Employee to whom an Option is granted.

          (n)  PLAN means this 1995 Employees' Stock Purchase Plan of the
               Company.

          (o)  RELATED CORPORATION means any corporation which is a parent
               corporation of the Company, as defined in Section 424(e) of the
               Code, and any corporation controlled by that parent corporation
               or the Company.

          (p)  STOCK means common stock, $0.10 par value, of the Company.

         2. PURPOSE OF THE PLAN. The Plan is intended to encourage ownership of
Stock by employees of the Company and to provide additional incentive for the
employees to promote the success of the business of the Company. It is intended
that the Plan shall be an "employee stock purchase plan" within the meaning of
Section 423 of the Code.

         3. TERM OF THE PLAN. The Plan shall become effective on December 18, 
1995. No option shall be granted under the Plan after November 30, 2005.

         4. ADMINISTRATION OF THE PLAN. The Plan shall be administered by the
Committee, which shall determine from time to time whether to grant Options
under the Plan, shall specify which dates shall be Grant Dates and Exercise
Dates, shall determine the fair market value of the Stock, and shall fix the
maximum percentage of each Optionee's Compensation which may be withheld for the
purpose of purchasing shares of Stock; PROVIDED, that, the maximum percentage
shall not exceed five percent. The Committee shall have authority to interpret
the Plan, to prescribe, amend and rescind rules and regulations relating to the
Plan, to determine the terms of Options granted under the Plan, and to make all
other determinations necessary or advisable for the administration of the Plan.


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         5. TERMINATION AND AMENDMENT OF PLAN. The Committee may terminate or 
amend the Plan at any time; PROVIDED HOWEVER, that the Committee may not,
without approval by the holders of a majority of the shares of Stock, increase
the maximum number of shares of Stock purchasable under the Plan, change the
description of employees or classes of employees eligible to receive Options,
change the manner of determining the exercise price of Options, or extend the
period during which Options may be granted or exercised. No termination of or
amendment to the Plan may adversely affect the rights of an Optionee with
respect to any Option held by the Optionee as of the date of such termination or
amendment.

         6. SHARES OF STOCK SUBJECT TO THE PLAN. No more than an aggregate of
250,000 shares of Stock may be issued or delivered pursuant to the exercise of
Options granted under the Plan, subject to adjustments made in accordance with
Section 9.8. Shares to be delivered upon the exercise of Options may be either
shares of Stock which are authorized but unissued or shares of Stock held by the
Company in its treasury. If an Option expires or terminates for any reason
without having been exercised in full, the unpurchased shares subject to the
Option shall become available for other Options granted under the Plan. The
Company shall, at all items during which Options are outstanding, reserve and
keep available shares of Stock, sufficient to satisfy such Options, and shall
pay all fees and expenses incurred by the Company in connection there with. In
the event of any capital change in the outstanding Stock as contemplated by
Section 9.8, the number of shares of Stock reserved and kept available by the
Company shall be appropriately adjusted.

         7. PERSONS ELIGIBLE TO RECEIVE OPTIONS. Each employee of the Company 
or a specified Related Corporation shall be granted an Option on each Grant Date
on which such employee meets all of the following requirements:

          (a)  The employee has been employed by the Company and the Related
               Corporation for at least one month and is customarily so employed
               by the Company for at least twenty hours per week and for more
               than five months per calendar year.

          (b)  The employee will not, after grant of the Option, own stock
               possessing five percent or more of the total combined voting
               power or value of all classes of stock of the Company or of any
               Related Corporation. For purposes of this paragraph (b), the
               rules of Section 424(d) of the Code shall apply in determining
               the stock ownership of the employee, and stock which the employee

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               may purchase under outstanding options shall be treated as stock
               owned by the employee.

          (c)  Upon grant of the Option, the employee's rights to purchase stock
               under all employee stock purchase plans (as defined in Section
               423(b) of the Code) of the Company and its Related Corporations
               will not accrue at a rate which exceeds $25,000 of fair market
               value of the stock (determined as of the Grant Date) for each
               calendar year in which such option is outstanding at any time.
               The accrual of rights to purchase stock shall be determined in
               accordance with Section 423(b)(8) of the Code.

         8.    DATES FOR GRANTING OPTIONS. Options shall be granted on each date
designated by the Committee as a Grant Date.

         9.    TERMS AND CONDITIONS OF OPTIONS.

          9.1  GENERAL. All Options granted on a particular Grant Date shall
               comply with the terms and conditions Set forth in Section 9.3
               through 9.12, and each Option shall be identical except as to the
               number of shares of Stock purchasable under the Option, which
               shall be determined in accordance with Section 9.2.

          9.2  NUMBER OF SHARES. The maximum number of shares of Stock which an
               Optionee shall be permitted to purchase shall be an amount equal
               to five percent of the Optionee's Compensation as of the Grant
               Date divided by 85 percent of the Market Value of the Stock as of
               the Grant Date.

          9.3  PURCHASE PRICE. The purchase price of shares of Stock shall be 85
               percent of the lesser of (a) the Market Value of the shares as of
               the Grant Date, or (b) the Market Value of the shares as of the
               Exercise Date, or such greater percentage as may be set by the
               Committee from time to time.

          9.4  RESTRICTIONS ON TRANSFER. Options may not be transferred
               otherwise than by will or under the laws of descent and
               distribution. An Option may not be exercised by anyone other than
               the Optionee during the lifetime of the Optionee. Shares of Stock
               may be sold or otherwise transferred by the Optionee without
               restriction subject to the provisions of Section 9.11 and the
               Stock Purchase Agreement that will be signed pursuant to Section
               9.10.

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          9.5  EXPIRATION. Each Option shall expire at the close of business on
               the Exercise Date or on such earlier date as may result from the
               operation of Section 9.6.

          9.6  TERMINATION OF EMPLOYMENT OF OPTIONEE. If an Optionee ceases for
               any reason (other than death or retirement) to be continuously
               employed by the Company or a Related Corporation, whether due to
               voluntary severance, involuntary severance, transfer, or
               disaffiliation of the employer corporation with the Company, his
               or her Option shall immediately expire, and the Optionee's
               accumulated payroll deduction shall be returned by the Company
               without interest. For purposes of this Section 9.6, an Optionee
               shall be deemed to be employed throughout any leave of absence
               for military service, illness or other bona fide purpose which
               does not exceed the longer of ninety days of the period during
               which the Optionee's reemployment rights are guaranteed by
               statute or by contract. If the Optionee does not return to active
               employment prior to the termination of such period, his or her
               employment shall be deemed to have ended on the ninety-first day
               of such leave of absence.

          9.7  RETIREMENT OR DEATH OF OPTIONEE. If an Optionee retires or dies,
               the employee or in the case of death, his or her Beneficiary,
               shall be entitled to withdraw the Optionee's accumulated payroll
               deductions without interest or to purchase shares on the Exercise
               Date to the extent that the Optionee would be so entitled had he
               or she continued to be employed by the Company. The number of
               shares purchasable shall be limited by the amount of the
               Optionee's accumulated payroll deductions as of the date of his
               or her retirement or death. Accumulated payroll deductions shall
               be applied by the Company toward the purchase of shares only if
               the Optionee or Beneficiary submits to the Company a Stock
               Purchase Agreement pursuant to Section 9.10. Accumulated payroll
               deductions not withdrawn or applied to the purchase of shares
               shall be delivered by the Company to the Optionee or Beneficiary
               without interest within a reasonable time after the Exercise
               Date.

          9.8  CAPITAL CHANGES AFFECTING THE STOCK. In the event that, between
               the Grant Date and the Exercise Date of an Option, a stock
               dividend is paid or becomes payable in respect of the Stock or
               there occurs a split up or contraction in the number of shares of
               Stock, the number of shares for which the Option may thereafter


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               be exercised and the price to be paid for each such share shall
               be proportionately adjusted. In the event that, after the Grant
               Date, there occurs a reclassification or change of outstanding
               shares of the Stock or a consolidation or merger of the Company
               with or into another corporation or a sale or conveyance,
               substantially as a whole, of the property of the Company, the
               Optionee shall be entitled on the Exercise Date to receive shares
               of Stock or other securities equivalent in kind and value to the
               shares of stock he or she would have held if he or she had
               exercised the Option in full immediately prior to such
               reclassification, change, consolidation, merger, sale or
               conveyance and had continued to hold such shares (together with
               all other shares and securities thereafter issued in respect
               thereof) until the Exercise Date. In the event that there is to
               occur a recapitalization involving an increase in the par value
               exceeding the exercise price under an outstanding Option, the
               Company shall notify the Optionee of such proposed
               recapitalization immediately upon its being recommended to the
               Company's shareholders, after which the Optionee shall have the
               right to exercise his or her Option prior to such
               recapitalization; if the Optionee fails to exercise the Option
               prior to recapitalization, the exercise price under the Option
               shall be appropriately adjusted. In the event that, after the
               Grant Date, there occurs a dissolution or liquidation of the
               Company, except pursuant to a transaction to which Section 424(a)
               of the Code applies, each Option to purchase Stock of the Company
               to be dissolved or liquidated shall terminate, but the Optionee
               holding such Option shall have the right toe exercise his or her
               Option prior to such dissolution or liquidation.

          9.9  PAYROLL DEDUCTIONS. An Optionee may purchase shares under his or
               her Option by completing and returning to the Company a
               Membership Agreement indicating the amount of his or her
               Compensation, not to exceed five percent, which is to be withheld
               each pay period. A Membership Agreement may continue from the
               period following one Grant Date to the periods following
               subsequent Grant Dates until revoked by the Optionee. The
               Optionee may withdraw any or all of his or her accumulated
               payroll deductions on the Exercise Date or such earlier date as
               is permitted by the Membership Agreement by submitting a written
               request therefor to the Company no later than two weeks prior to
               the date on which the withdrawal will be effective.

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          9.10 EXERCISE OF OPTIONS. On the Exercise Date the Optionee may
               purchase the number of shares purchasable by his or her
               accumulated payroll deduction, provided that:

               (a)  The number of shares of Stock purchasable shall not exceed
                    the number of shares the Optionee is entitled to purchase
                    pursuant to Section 9.2.

               (b)  If the number of shares purchasable includes a fraction,
                    that number shall be adjusted to the next smaller whole
                    number and the purchase price shall be adjusted accordingly.

               The Optionee may elect to purchase shares of Stock from his or
               her Base Amount by completing and returning to the Company a
               Stock Purchase Agreement no later than two weeks prior to the
               Exercise Date. If the Company does not receive a Stock Purchase
               Agreement from the Optionee by such date, accumulated payroll
               deductions will be returned on the Exercise Date without
               interest.

          9.11 DELIVERY OF STOCK. Within a reasonable time after the Exercise
               Date, the Company shall deliver or cause to be delivered to the
               Optionee a certificate or certificates for the number of shares
               purchased by the Optionee. At the time of any exercise of any
               option, the Company may, if it shall deem it necessary or
               desirable for any reason connected with any law or applicable
               regulation of the Securities and Exchange Commission or state
               securities laws, require the Optionee or a transfer of the
               Optionee's rights to represent in writing to the Company that it
               is such person's then intention to acquire the Stock for
               investment and not with a view to the distribution thereof. Such
               representation shall lapse when in the view of the Company it is
               no longer necessary under the law or regulations in existence at
               the time. The Company shall have the right to place a legend on
               all certificates that the shares represented by such certificates
               may not be transferred unless a Registration Statement with
               respect to these shares is effective under the Securities Act of
               1933, as amended, or unless the Company shall receive an opinion
               of counsel satisfactory to it that transfer will not violate said
               act or regulations thereunder. If any law or applicable
               regulation of the Securities and Exchange Commission or other
               body having jurisdiction in the premises shall require that the
               Company or the Optionee take any action in connection with the

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               shares being purchased under the Option, delivery of the
               certificate or certificates for such shares shall be postponed
               until the necessary action shall have been completed. The
               Optionee shall have no rights as a shareholder in respect of
               shares for which he or she has not received a certificate.

          9.12 RETURN OF ACCUMULATED PAYROLL DEDUCTIONS. In the event that the
               Optionee or the Beneficiary is entitled to the return of
               accumulated payroll deductions, whether by reason of voluntary
               withdrawal, termination of employment, retirement, death, or in
               the event that accumulated payroll deductions exceed the price of
               share purchased, such amount shall be returned without interest
               within a reasonable time after the Exercise Date or such earlier
               date as is permitted by the Membership Agreement. Payroll
               deductions shall be returned by the Company to the Optionee or
               the Beneficiary, as the case may be. An Optionee's Membership
               Agreement may specify that amounts exceeding the purchase price
               will be carried forward to the next option period under the Plan.