[en] Circular stapling instruments for cutting and applying one or more surgical staples to tissue are disclosed. The instruments include various forms of anvil locking systems designed to selectively prevent the anvil from moving axially relative to the stapling head of the stapling instrument. One embodiment employs an elongated gear rack that is selectively engagable with locking gears to prevent axial motion of the elongated gear rack and an adjustment shaft used to axially position the anvil. Other embodiments employ a gear assembly that cooperates with the elongated gear rack. A locking member is movably supported by a handle assembly that houses the gear assembly and elongated gear rack. The locking member is configured for selective meshing engagement with the gear assembly to ultimately prevent axial movement of the gear rack and adjustment shaft.