Abstract: A known thermal actuator includes a capsule, which contains a diaphragm controlling the position of a stem. A thermal expansion substance like wax is contained in the capsule which operates as a thermostat in a thermostatically controlled valve such as a thermostatically controlled mixing valve. The stem of the actuator is connected with a valve body movable between two valve seats for a hot and cold medium, respectively, and the capsule is placed in the area of the mixed medium for transmitting the heat of the mixed medium to the thermal expansion substance. In order to present a simple and more effective actuator that can react promptly at changes of temperature and which does not require an extra temperature sensor, the capsule includes an oblong cuplike sensor body, the surface of which is at least partly deformed in such a way that its thermal transmittance area is increased.