Abstract: An actuator for imparting a series of stepwise rotational movements to a valve shaft (2) or the like is provided. A bidirectional actuator, comprised of two opposing unidirectional actuators, includes a central drive wheel (6) secured to the shaft (2) and a pair of drive pistons (8, 10) axially reciprocable along the shaft (2). During reciprocation of the drive pistons (8, 10) protruding follower pins (30, 32) move within cam races (44, 46) disposed in the fixed housing (4). The cam races (44, 46) are so configured to allow the drive pistons (8, 10) to move into engagement with the drive wheel (6), before imparting a stepwise rotational movement thereto. During the return motion of the drive pistons (8, 10) the cam races (44, 46) are so configured to allow the pistons to disengage the drive wheel (6) before returning to their starting positions. A detent (54) for holding the drive wheel (6) and shaft (2) in any one of a preferred plurality of index positions is also disclosed.