A surgical instrument, comprising: a housing including an elongated shaft having a distal portion and a proximal portion coupled to the housing, the elongated shaft defining a longitudinal axis and a mandrel at the proximal portion; an inner shaft member extending at least partially through the elongated shaft; an actuating mechanism operably coupled to the mandrel and configured to selectively cause movement of the elongated shaft in a longitudinal direction with respect to the inner shaft member; and an end effector adapted for treating tissue and including an upper jaw member pivotally coupled to a distal portion of the inner shaft member about a pivot axis and a lower jaw member supported by the distal portion of the elongated shaft, the elongated shaft configured to pivot the upper jaw member relative to the lower jaw member upon longitudinal movement relative to the inner shaft member, characterised in that the elongated shaft has a generally u-shaped profile including opposing interior sidewalls (162a, 162b), and wherein the inner shaft member has a generally u-shaped profile including opposing exterior sidewalls (182a, 182b) disposed laterally outward from the opposing interior sidewalls (162a, 162b) of the elongated shaft, further comprising a knife disposed between the opposing interior sidewalls (162a, 162b) of the elongated shaft and selectively movable in a longitudinal direction with respect to the elongated shaft, wherein the interior sidewalls (162a, 162b) of the elongated shaft are spaced apart and receive the knife therebetween and serve as a knife guide for the knife during longitudinal reciprocation of the knife to divide tissue grasped between the jaw members. a housing including an elongated shaft having a distal portion and a proximal portion coupled to the housing, the elongated shaft defining a longitudinal axis and a mandrel at the proximal portion; an inner shaft member extending at least partially through the elongated shaft; an actuating mechanism operably coupled to the mandrel and configured to selectively cause movement of the elongated shaft in a longitudinal direction with respect to the inner shaft member; and an end effector adapted for treating tissue and including an upper jaw member pivotally coupled to a distal portion of the inner shaft member about a pivot axis and a lower jaw member supported by the distal portion of the elongated shaft, the elongated shaft configured to pivot the upper jaw member relative to the lower jaw member upon longitudinal movement relative to the inner shaft member, characterised in that the elongated shaft has a generally u-shaped profile including opposing interior sidewalls (162a, 162b), and wherein the inner shaft member has a generally u-shaped profile including opposing exterior sidewalls (182a, 182b) disposed laterally outward from the opposing interior sidewalls (162a, 162b) of the elongated shaft, further comprising a knife disposed between the opposing interior sidewalls (162a, 162b) of the elongated shaft and selectively movable in a longitudinal direction with respect to the elongated shaft, wherein the interior sidewalls (162a, 162b) of the elongated shaft are spaced apart and receive the knife therebetween and serve as a knife guide for the knife during longitudinal reciprocation of the knife to divide tissue grasped between the jaw members.