Abstract: A locking device for connectors or closures which device prevents rotation of the means for holding a pair of members in engagement wherein the locking device includes a shield which overlies all surfaces on the locking device which might be engaged by a wrench to rotate the locking device while the closure is subjected to pressure. The locking device includes a pressure responsive member exposed to pressure within the closure which carries the shield and positions the shield to overlie all surfaces on the device on which a wrench may be placed and when there is no pressure differential across the pressure responsive member permits the shield to be moved to an out of the way position exposing a wrench surface.