A surgical stapling instrument suitable for performing a gastrointestinal anastomosis is provided. The stapling instrument incorporates an improved staple actuator mechanism operable by separate control buttons to accomplish its stapling and cutting actions in two stages. The two stage operation of the stapling instrument reduces the maximum force required to operate the instrument. The improved actuator mechanism permits selection of different staple heights to be produced by the same instrument. In addition, the actuator mechanism is designed to stabilize the upper and lower jaw members by providing longitudinal and lateral support along substantially the entire length of the operative portions of the jaw members. Preferably, the actuator mechanism comprises an elongated, I-beam structure slidably mounted for longitudinal movement relative to the jaw members. The I-beam assembly includes upper and lower flanges which provide both longitudinal and lateral support along substantially the entire length of the anvil and staple cartridge carrying portions of the jaw members. A sleeve slidably mounted on one of the I-beam flanges allows the spacing of the jaw members to be adjusted to produce one of two predetermined closed staple heights.