Abstract: A locking cover apparatus for underground fuel tanks includes a cover having a depending annular flange mountable over the open end of a fuel tank filler tube. A lock is mounted on a boss secured to the bottom surface of the cover and rotates a plate between a locked position and an unlocked position. A plurality of lock bars are pivotally attached to the plate and are extensible and retractable through apertures in the depending flange and apertures in the filler tube for attaching and releasing the cover from engagement with the filler tube. First and second cover members respectively cover the lock and the boss in which the lock is mounted on the cover. Specially configured keys are insertable into cavities formed in the first and second cover members to permit authorized removal of the cover members from the cover.