The present invention relates to downhole apparatus for limiting rotation of a rotor (4) relative to a stator (8) associated with said rotor (4). Apparatus according to the invention is provided with a locking member (12) movable from a first axial position to a second axial position by the application of a static fluid pressure to a differential area of said locking member (12). The apparatus is characterised by means for selectively applying static fluid pressure to said differential area of said locking member (12).
