Abstract: A self-locking system for use with circuit board holding frames comprises two side panels each having a specially configured slot for guiding a retainer bar, which extends between the two slots, between a locked and a loading position. Each slot has a shape similar to that of an upside down "L" and is positioned on each side panel at the top corner adjacent to the holding frame opening. In the locked position, the retainer bar, which is held in the guide slots by shoulder screws at each end, extends across the frame opening thereby blocking any outward board movement. In the loading position, the retainer bar rests out of the opening thus providing clearance for board removal or installation. A lip, which prevents the retainer bar from inadvertently toppling down into the locked position, is dimensioned so that the retainer bar may fall down into the locked position upon sufficient mechanical vibration if the system is mistakenly left unlocked.