Abstract: The present invention relates to an actuator for turning a shaft (11) to a limited degree, e.g. the shaft of a butterfly valve or the like, the actuator comprising a housing (2) having a cylindrical chamber (6), inlets and outlets (7, 8) for a pressure fluid to the cylindrical chamber, a piston (9) in the cylindrical chamber, which can be displaced in the cylindrical chamber by means of a pressure fluid and is provided with a rack (10) with a toothed segment (27) cooperating with a gear ring segment (22), fixedly mounted on said shaft (11), which extends right through the cylindrical chamber (6) and through the wall of the housing (2).