Abstract: A valve for connection in a pipeline or duct for the conveyance of a fluent medium includes a valve plate which is movable laterally relative to the duct between respective open and closed positions, with resilient sealing means provided on or in association with the valve plate. The sealing means is provided on an axially movable member to engage one face of the valve plate, while the opposed sealing means is carried by a fixed end member so that the valve plate is clamped between them. Either of the sealing means may be omitted. A ram actuates the valve plate by rotation of a shaft while simultaneously rotating a control ring forming part of the control mechanism which moves the member axially to clamp and release the valve plate.