Abstract: A single valve element disc and seat gate valve has an in-line flow passage therethrough. The valve seat plane is inclined approximately 5.degree. from the valve stem. First and second cam surfaces are positioned outside the flow passage, and are inclined approximately 30.degree. from the stem axis so that the cam surfaces and the valve seat plane define a wedge for receiving the valve disc and a hanger supporting the disc.The valve seat is a frustoconical surface. The valve disc is fixed to the hanger by means for urging the valve disc against the seat at the center thereof in a manner to ensure an effective seal therebetween. The gate valve includes a back seal for permitting the stem seal to be removed and replaced while the valve remains in service.