Abstract: A liquid tank anti-theft device for use in a tank filler tube of the type having an apertured plate fixed therein to limit the size of gas nozzle insertable therein. The device acts to prevent insertion of gas removal means, such as a siphon tube, therein. The device comprises a tubular member for insertion in the tank filler tube and includes a projection or flange at the top of the member and also thread-like means for cooperatively engaging the apertured plate and preventing removal of the device from the tank filler tube. Insertion of a siphon tube is prevented by means such as a screen-like grid at the lower end of the tubular member.