Abstract: A butterfly valve construction for use particularly on pipelines through which fluid under pressure is adapted to flow and to be selectively cut off. The butterfly valve disc is rotatably supported within the valve casing. Within the valve casing at a region whereat the valve disc is adapted to be turned for closing an elastomeric ring is supported and a plurality of clamp ring segments are supported about the casing periphery to abut one face of the elastomeric ring. Each clamp segment is adjustable to a fixed selective position by positively acting means to compress the elastomeric ring against an internal surface of the disc, thereby also extending the ring in a direction toward the valve disc edge when the latter is in a closed position. This extension is adequate to force the elastomeric ring to fit tightly to the valve ring at valve closing times and directly form a tight seal thereto.