Abstract: A shut-off valve in a gas line leading to a fuel tank of a motor vehicle, comprising a valve seat and a valve body which is movable by means of the opening and closing movement of a fuel tank lock. In order to provide a valve which opens a large flow cross-section in the open position, it is provided that the valve body comprises a flexible diaphragm which is pretensioned against the valve seat, an actuating lever being fastened at the side of the diaphragm remote of the valve seat, the free end of the actuating lever communicating with a tappet which is movable transversely relative to the longitudinal axis of the actuating lever and is coupled with the fuel tank lock. The valve seat is enclosed by an annular space which can be completely covered by the diaphragm, wherein the annular space communicates with a branch of the gas line and the interior space of the valve seat communicates with the other branch of the gas line.