Abstract: A diaphragm valve (1) comprising a valve body (2); a diaphragm (12) which is sealed to the valve body (2) to define a flow passage (9) which extends between an inlet port (7) and an outlet port (8), both defined by the valve body (2); and an operating mechanism secured to the valve body (2) for moving the diaphragm (12) into sealing engagement with a seat (11) provided on the valve body (2) in order to close the flow passage (9) to fluid flow. The valve body (2) and diaphragm (12) are formed as a disposable assembly having a relatively less flexible region which forms a valve seat (11) and a relatively more flexible region (12) which forms the diaphragm which may be forced into engagement with the valve seat (11) to close the flow passage (9) to fluid flow, and a housing (3) is provided for mechanically supporting the region of the valve body in which the seat area is defined.