Abstract: In a preferred ball-valve construction, a valve body has spaced inlet and outlet ports on an axis of flow that is controlled by an interposed ball; the ball is rotatable about a body-fixed axis which intersects the axis of flow at the geometric center of the ball. In the closed condition of the valve, an annular valve-seat member associated with one of the ports has circumferentially sealed contact with the ball. The annular valve-seat member is guided for axial displaceability into and out of its engageability with the ball, and actuation of the valve utilized coordination of valve-seat displacement with ball rotation such that valve-seat engagement with the ball is only for the closed condition of the valve, and such that, valve-opening rotation of the ball can occur only while the annular valve-seat member is in axial-clearance relation with the ball. Various embodiments are disclosed.