A locking, threaded gas cap for vehicle fuel tanks having threaded filler necks, the cap comprising a closure member threaded to engage and close the filler neck and an outer shell member mounted on the closure member for relative rotation to provide a hand grip. The closure member provides a plurality of abutments spaced peripherally about its axis while the shell member provides at least one yieldably movable drive finger extending radially outwardly into engagement with the abutments. The shell member provides a first stop for preventing movement of the drive finger out of engagement with the abutments when the shell member is rotated in one direction to thread the closure member onto the filler neck. The shell member provides a second stop having an operative position for preventing movement of the drive finger out of engagement with the abutments when the shell member is rotated in the opposite direction to remove the closure member. A key-operated lock is mounted in the shell member and connected to the second stop to move the second stop into and out of its operative position.