a catch mechanism for installation in a door frame to cooperate with a door lock on a fire door . the mechanism includes a catch member pivotable between an operative position in which to serve as a door catch and an inoperative position to enable release of the door no matter what the condition of the door lock . the mechanism includes a heat responsive locking device which locks the catch member in its catch position in the event of a fire generating high temperatures . the locking device may comprise a biased locking plunger mounted in a cavity within the catch member and normally held in a retracted position by a heat fusible metal plug but extendible on melting of the plug to engage the catch body and so lock the catch member .