Abstract: The invention is directed to a pneumatic or hydraulic rotational power drive particularly suited for a ball stopcock type valve, the power drive consisting of a power cylinder being located coaxially to the rotation axis of the cock plug, the piston rod stroke of which is convertible, via a spindal drive, to the rotary movement of a driving member being positively engaged with the cock plug. In such arrangement, the power cylinder may be designed comprising a piston or diaphragm, and for double action, i.e., working in either direction or working solely unidirectional while being reset by means of spring action.