Abstract: A ball valve of the type comprising a housing having a ball mounted for 90.degree. rotation therein between open and closed valve positions utilizes a ball whose exterior surface is shaped to define two noncoaxial hemispherical surface portions of like size and shape that are integrated with one another and offset from one another along a plane that is oriented at substantially 45.degree. to an axially directed bore in the ball. The hemispherical surface portions of the ball are in touching engagement with a pair of annular seat rings in the housing in the open position of the valve, and engage the seat rings with increasing force as the ball is rotated from the open position to the closed position of the valve.