A security device, particularly for a motor vehicle, which is adapted to isolate the fuel supply from the engine of the vehicle is disclosed. The security device comprises a first body member locatable at one side of a vehicle floor and a second body member locatable at an opposite, second side of said floor, said body members being securable together through said floor by securing means located at said second side, and engaging means extending from a lower surface of said first body member, a rotatable locking device located in said first body member and a rotatable valve member located in said second body member and being movable, upon rotation of said locking device, from a first position allowing fuel to flow through said valve member to a second position preventing such flow, the security device being devoid of any release means at said one side of said vehicle floor.