A disposable surgical stapling instrument for the joining together of tubular body organs such as the organs of the alimentary canal. The instrument comprises an elongated body having a tubular housing portion and a coaxial cylindrical handle portion. The tubular housing portion supports at its free end a coaxial cylindrical casing containing at least one annular array of surgical staples. An adjusting rod is coaxially and non-rotatively mounted within the instrument body and has a first end extending beyond the staple-containing casing. An anvil, of improved configuration for easier removal from the anastomotic site, is removably mounted by means of an anvil nut on the first end of the adjusting rod facing the staple-containing casing. By means of an adjustment knob at the free end of the instrument body handle portion, the anvil and the adjusting rod are shiftable axially of the instrument body between a position wherein the anvil is adjacent the staple-containing casing and a position wherein the anvil is spaced from the staple-containing casing. The anvil has a plurality of anvil pockets formed therein to clinch the staples over a range of distances between the anvil and the staple casing constituting a working gap. The adjustment rod has a second end mounting an indicator cooperating with the adjustment knob to give both visual and tactile indications that the anvil is spaced from the staple casing within the operating gap. A ring scale is mounted on the adjusting knob enabling accurate setting of the distance between the anvil and the staple casing, within the working gap, according to the thickness of the tissue of the tubular body organs being joined. The instrument is also provided with a lever actuated staple driver and scalpel assembly and a slide safety lockout to prevent undesired actuation of the lever.