The mechanism is applicable to a surgical tool which has a jaw (57) pivotably mounted to a distal end of a longitudinally extending support (58). The jaw is operated by action on a rearwardly facing cam follower (52, 53) by a lobe (51) on the distal end of an actuating tube (50). Rotation of the actuating tube (50) causes pivoting of the jaw (57) into and out of operative relationship with another jaw member (21) of the tool. The mechanism may be applied to any manipulative or gripping type of surgical implement, but is particularly applicable to ultrasonic tools adapted for cutting and/or coagulating.