Abstract: A lock assembly for automatically dead bolting a closure to a closure frame when the ambient temperature is raised above a predetermined temperature by fire comprises a striker plate, which defines a bolt hole, mounted on the closure frame. A cooperating latching mechanism, installed in the closure, includes a housing and a bolt assembly mounted in housing for reciprocal movement to and from a latched position projecting into the striker plate bolt hole and for movement to an extreme dead bolted position projecting further into the bolt hole beyond the latched position. A stop pin mechanism associated with the bolt assembly prevents the bolt assembly from being moved out of the dead bolted position once this position is assumed and a spring urges the bolt toward the dead bolted position.