Abstract: An actuator shaft heat-sensitive locking apparatus is disclosed comprising a cylindrical housing having an axial bore. A shaft link is provided in the bore and is adjustable to contact gate valve actuator shafts of varying lengths. Once adjusted, the shaft link is movable axially between a first position and a second position, allowing the actuator shaft to close the gate valve. There is also provided a means movably extending through the housing for partially blocking the bore and a fusible element slidably mounted in the bore between the shaft link and the means for partially blocking the bore. The fusible element has both a solid state and a heat responsive state. In its solid state, the fusible elemnt prevents movement of the shaft link by enaging the bore blocking means; in its heat responsive state, the fusible element allows movement of the shaft link between the first position and second position.