A penile prosthesis is provided, which includes a generally cylindrical body portion comprising a cavity with an inner wall, a proximal end, a distal end, and at least one volume-displacing member occupying at least a portion of the cavity, wherein the at least one volume-displacing member is smaller in at least one dimension than the dimensions of the cavity to provide an internal chamber at least partially surrounding the at least one volume-displacing member and adjacent to the inner wall of the cavity. The prosthesis further includes a fluid transfer tube extending from the body portion for transferring fluid to and from the internal chamber, wherein the device is convertible from a flaccid condition to a rigid condition when a sufficient volume of fluid is transferred into the internal chamber to substantially fill the internal chamber.