Abstract: What is proposed by the invention is a fin deployment mechanism for missiles in which a fin member can be extended from or retracted to the contour of a missile by means of a rotary to linear motion transmission system employing a rotatable screw shaft having respective adjacent portions embodying a left hand and a right hand thread for supporting respective ones of a pair of screw nuts, each of which are provided with a cam slot for receiving a cam follower mounted on the fin member, or fin supporting member, so that when the screw shaft rotates the screw nuts are caused to move therealong in mutually opposite directions and via the cooperation between the cam slots and the cam followers the fin is caused to rotate through a given angle. In each of the extreme positions of the fin member a locking engagement is effected for retaining such positions in a locked condition.