Abstract: An actuator (1) in particular for proportionally influencing the fluid flow in hydraulic and/or pneumatic devices, including a cylindrical housing (2) with an elongated actuating element (3) movably supported therein, elements (11, 12, 13) of memory metal, engaging said actuating element (3), the housing (2) and the actuating element (3) movable relative to one another by activation of the elements (11, 12, 13) of memory metal, if necessary against the force exerted by an element (15) of permanent resilient material. The housing (2) is provided at one end (8) with an opening (9) through which the actuating element (3) protrudes longitudinally upon relative movement. Bringing about the intended proportional operation, the actuator (1) includes locking means (16, 17, 19), engaging the actuating element (3), for mutually locking the actuating element (3) and their housing (2) in one or more positions.