A locking gas cap for a threaded filler neck having a sealing lip extending peripherally about its axis, the cap comprising a closure member threaded to engage and close the filler neck, a gasket on the closure member for engaging and sealing against the lip, an outer shell providing a hand grip secured to the closure member for rotation thereon, a key-operated lock and at least one lock-actuable bolt connected to the shell for rotation therewith. A race is disposed for rotation in the shell, and a driving connection is provided between the race and the closure member which is torque limited in the direction which advances the closure member on the filler neck and positive in the cap removal direction. A spring is provided for yieldably urging the bolt into engagement with the race. The spring cooperates with a plurality of engaging pockets in the race to provide a positive driving connection between the shell and race in the direction which advances the closure onto the filler neck. When the lock is not in a cap removal position and the shell is rotated in the cap removal direction, the bolt cams out of the pockets against the urging of the spring, allowing the shell to turn with respect to the race to prevent cap removal. When the lock is in its cap removal position, it blocks motion of the bolt out of the pockets to lock the shell to the race.