Abstract: A fluid displacement device is described which may be used as a pump, a motor or a device such as a vibrator for concrete. It includes a body having at least three, and preferably four or five wall portions, which are disposed one after the other along the axis of the body, discrete chambers being formed behind each wall portion, which may, for example, be a toroidal diaphragm, means being provided to feed an operating fluid under pressure to the discrete chambers to move the wall portions transversely to the axis of the body in a sequence to cause displacement of an external fluid axially of the body. When used as a motor, movement of the external fluid axially of the body will cause displacement of the wall portions and thus flow of the operating fluid so that power may be derived therefrom.