Abstract: The invention concerns an actuator for the operation of on-off valves. Said actuator includes a supporting body (1) in which there is transversally disposed at least one pneumatic cylinder (8, 9) which is capable of activating in two directions, a rack (7) which in turn is engaged with a cogged wheel (31; 50; 100) co-axial with the central shaft (15, 66, 104) of said actuator and able to transmit its own rotation through a pair of free wheels (27, 28; 61, 62; 109, 110) which rigidly and alternatively engage themselves in discordant directions with a ring (33, 34; 62, 63; 108) integral with the central shaft of the actuator, and pneumatic pistons for achieving the relative axial movement between the cogged wheel and the shaft of the actuator. Depending on the mutual distance between the free wheels, and in the absence of the pneumatic signal, the actuator has its central shaft blocked or manually maneuvereable.