Abstract: A locking device for fuel tanks which blocks the filler neck in the tank. A cylindrical sleeve having an opening through its length inserts into the filler neck below the outer end of the filler neck and is secured by set screws threaded through the sleeve wall into contact with the filler neck. Blocking means comprising a cylinder lock and an annular adapter, when the device is used on large diameter filler necks, fit into the sleeve, blocking the opening in the sleeve and covering access to the set screws. The cylinder lock, which is key-activated, includes an arm which rotates into engagement with the sleeve, preventing removal of the blocking means.