Abstract: A tool is disclosed for shifting the sleeve of a port collar in a well casing, to open and close the ports in the collar. The basic tool is made up of a housing having a double piston assembly mounted slideably in a transverse bore in the housing. The transverse bore is intersected by a lengthwise bore in the housing. Fluid is carried into the housing from a tubing string connected into the lengthwise bore. The double piston assembly has an outside face in contact with fluid in the well casing, and an inside face in contact with fluid inside the housing bore. In addition, the outside face has a larger surface area than the inside face. This differential in the piston surface area allows the piston assembly to be held in a "retract" position by applying fluid pressure to the outside face. Alternatively, the piston assembly can be moved laterally to engage the port collar sleeve by applying fluid pressure to the inside face.