A diaphragm operated pressure regulator has a housing with a flow passage therethrough. The diaphragm in the housing is operatively connected to a ball valve in the flow passage for positioning the ball valve to control fluid flow through the flow passage at a predetermined pressure value. The diaphragm cooperates with the housing to define a displacement chamber. The displacement chamber is filled with fluid for backing up the diaphragm over its entire surface exposed to the displacement chamber. The diaphragm is preferably not perforated. A displacement piston cooperates with the displacement chamber to control the force applied to the diaphragm and thereby the flow through the flow passage. The displacement piston is adjustable into and out of the displacement chamber to vary the force applied to the diaphragm.