Abstract: A sliding door locking apparatus in which ratchet shaped catches project vertically from a housing and are depressible to a lowered position. The housing is horizontally mounted to the base of the door frame. One of the catches prevents the opening of the door by making contact with a blocking member that is attached to the slidable door panel. However, the door can be quickly closed because contact between the blocking member and the catch in the opposite direction depresses the catch, owing to its ratchet shape. When the catch is depressed to a lower position, the door can move freely in either direction. An alarm is provided which is responsive to an attempt to open the door when the catch is not depressed to a lowered position.