Abstract: A one-piece integral construction check valve molded of resinous plastic material is disclosed. The valve has a body and a cover including first and second cover portions. The first cover portion is hingedly connected to the body and the second cover portion in turn connected to the first cover portion. The cover portions are spaced from each other by a molded recess above a hinge connection which through plastic memory acts to downwardly bias the second cover portion into sealing engagement with the upper surface of the body portion so as to normally seal a fluid transmitting opening disposed therethrough into a closed position. Differential pressure within the passageway in which the valve is adapted to be mounted serves to hingedly pivot the second cover portion upwardly about the first portion to uncover the opening and therefore open the valve.