A surgical instrument including a closure system configured to move an anvil of an end effector between an open position, a partially closed position, and a closed position. The surgical instrument can further include a lock member configured to selectively engage and lock the closure system when the anvil is positioned in at least one of its partially closed and closed positions. In at least one embodiment, the surgical instrument can include a trigger configured to pivot the anvil, the lock member can include a follower portion, and the closure drive can include a lock spring configured to bias the follower portion against a cam surface of the trigger such that the follower portion can engage a notch in the cam surface when the anvil is pivoted into its partially closed position, or closed position, and prevent the anvil from being pivoted into its open position.
