Automatic locking device for fuel tanks

The device has a closure unit (12) swingably locked between an open position and a closing position for sealingly closing a tank filler neck (10). A rotatably driven bayonet ring (22) is arranged at the tank filler neck and locked in the closing position using a drive (36). The drive is held at the tank filler neck. The drive has a pinion (38), which is meshed with a toothed ring (40) on a periphery of the bayonet ring. A cam is formed at the bayonet ring, and controls closing movement of the closure unit.