Abstract: A plug valve has a body with inlet and outlet ports with a cylindrical chamber therebetween for receiving a cylindrical plug. The body includes a circumferential resilient seat ring around the outlet port The cylindrical plug includes a generally horizontal “U” shaped transaxially cut-out portion which aligns with the inlet and outlet ports when in the open position, where the cross section opposite the cut-out portion is the shape of a segment of a circle. When the plug is rotated to the closed position, the segment covers the outlet port and the circumferential resilient seat. The plug includes a generally rectangular slot across the top surface of the plug at right angles to the segment. The plug is rotated by a separate stem which has a corresponding end to that of the slot. In the closed posidon, the plug is able to move relative to the stem in the direction of flow. Thus, the upstream pressure will force the plug into the seat causing the seal to become tighter as the pressure rises.