Abstract: An adjustable flow control valve having a rectangular flow control member selectively positionable within a fluid passageway. The flow control member is mounted transversely to the flow passageway in a guide slot integrally formed in the valve housing with the longitudinal edges of the control member extending outwardly from the slot. A closed end cylindrical adjustment nut having a tubular portion surrounding one end of the mounted control member is guided for rotation in a housing bore. The internal diameter of the adjustment nut is threaded and engages corresponding external threads located on the longitudinal edges of the rectangular flow control member. Selective rotation of the adjustment nut is effective for positioning the flow control member into the fluid passageway transversely to the direction of fluid flow therethrough, thereby varying the effective fluid flow area.