A surgical instrument that includes an instrument shaft having proximal and distal ends, a tool disposed from the distal end of the instrument shaft, a control handle disposed from the proximal end of the instrument shaft, a distal motion member for coupling the distal end of the instrument shaft to the tool, a proximal motion member for coupling the proximal end of the instrument shaft to the handle, actuation means extending between the distal and proximal motion members for coupling motion of the proximal motion member to the distal motion member for controlling the positioning of the tool, a rotation knob disposed adjacent the control handle and rotatable relative to the control handle for causing a corresponding rotation of the instrument shaft and tool and a locking mechanism for fixing the position of the tool at a selected position and having locked and unlocked states. The rotation knob has a first position in which the locking mechanism is controlled to be in its locked state and a second position in which the locking mechanism is released to its unlocked state so as to allow tool positioning.