Abstract: An actuator (14) designed to remain without maintenance down a well for a prolonged period comprises at least one sealing bellows (44) and preferably a compensation bellows (46) between the well fluid and an internal chamber (30) that is full of hydraulic fluid. The bellows (44, 46) serve to make it possible either to omit dynamic elastomer gaskets altogether or at least to protect them from the well environment. The actuator (14) e.g. of the electromechanical type, can serve in particular to control a valve (12) having an opening of adjustable size. Both bellows (44, 46) are advantageously made of stainless steel. They can be mounted end-to-end or they can be quite separate.