Abstract: The check valve comprises an integral body of elastomeric material of generally exterior hemispherical shape with an annular flange portion on the base of the hemisphere. An inlet passage is formed in the base and terminates within the hemispherical portion, there being provided a cut from the end of the inlet passage to the outer periphery of the hemispherical portion defining a slit of given dimensions. This slit is formed at an acute angle to the plane of the base of the hemisphere; that is, the plane normal to the axis of the inlet passage. The geometry of the configuration is such that the slit will open to pass fluid either liquid or gas from the inlet to the exterior of the body when a differential pressure of the fluid across the body is exceeded and collapse or close when the differential pressure reverses to block reverse flow of fluid through the body.