Abstract: An apparatus for interlocking horizontally adjacent first and second circuit breakers and preventing the circuit breakers from simultaneously being in a closed operating position. The circuit breakers each have a pivotable actuating handle movable between a closed position which is proximal to the other circuit breaker, and a open position which is distal to the other circuit breaker. A housing lies between the first and second circuit breakers and an interlock bar is slidably retained in the housing. In operation, the interlock bar is slidable between a first position where the interlock bar prevents the first circuit breaker handle from pivoting into the closed position and a second position where the interlock bar prevents the second circuit breaker handle from pivoting into the closed position.