Abstract: A gate valve structure for use in mains carrying fluids, the gate valve structure being provided with a casing having a through-bore and a chamber communicating with the through-bore and in which a gate member reciprocates between closed and opened positions of the valve. The valve casing is split into two body members on a plane normal to the axis of the through-bore and one of the body members is arranged to contain all of the functional parts of the valve, including the valve seat, gate member, and actuating means for the gate member. By such an arrangement, the valve seat surrounding the through-bore in the one body member may be easily machined by conventional machining methods and the internal surfaces of both body members may be easily treated to make them non-corrosive.