Abstract: A rotary actuator for converting reciprocating motion to rotary motion including a housing having spaced-apart end plates connected by guide rods. A bellows is provided having opposite sides one of which is connected to one of the end plates, the bellows being adapted to be connected to a source of fluid under pressure so that the bellows expands from a collapsed to an expanded position. A guide plate is provided movably mounted on the guide rods with the other bellows side being connected thereto. A rack and pinion assembly is provided mounted on the other end plate, the rack being connected to the guide plate so that expansion and contraction of the bellows causes linear movement of the rack thereby imparting rotary movement to the pinion. A spring acts between the guide plate and the other end plate thus resisting expansion of the bellows and returning it to the collapsed position upon the release of fluid pressure.