Abstract: A nonreturn valve for use in controlling the flow of medical fluid. The valve has two mating valve shells which carry and communicate with first and second pipe connectors, respectively. A diaphragm with a protrusion on its periphery is held between the valve shells. Circumferentially disposed retaining formations on each of the valve shells causes pretension to occur in the diaphragm upon assembly of the valve shells. The axial location of the retaining formation on the valve shell which carries the valve seat is such that tension in the diaphragm lightly urges the diaphragm into sealing engagement with the valve seat, whereby a seal exists when there is no significant pressure on either side of the diaphragm.