Abstract: A ladder extension lock that serves as a back-up unit for an existing ladder lock system. The inventive lock releasably locks in place a section of a multiple section extension ladder. The lock is comprised of a control bar (20) planar with a rung on the fly section, having pivot pins (24) on each end. A pair of hooks (26) are attached to the pivot pins (24) and mounting blocks (28) are fastened to the side rails of the ladder and also the hooks (26). A pair of springs (37) are disposed between the bar (20) and the rung, urging the bar away from the rung. A pully (38), attached to a yoke (36), allows a halyard (46) to be engaged within a groove in the pulley (38). When an operator applies force upon the halyard (46), the bar (20) is forced upward rotating the hooks (26) to clear the rungs of the ladder while hoisting, and when pressure is quickly released, the hooks (26) rotate to their locking position and mate with the rung, creating a fail-safe mechanical lock between the rung and the side rails.