Abstract: A rotary flow control valve including a valve spool which is rotated back and forth to move a port in the valve spool into and out of alignment with the main fluid passageway of the valve to selectively open and close the valve. The spool has a pinion gear at each end of the spool and each pinion gear is engaged by a pair of racks with the racks in a pair moving at any given time in opposite directions. Each rack is constituted by spaced rack teeth formed in the side wall of a piston slidably mounted in a cylinder bore in the valve body, and the rotation of the valve spool is achieved by selective routing of pressurized fluid to the cylinder bores containing the pistons defining the racks.