Abstract: Provided is a damper for controlling air flow through an air duct. The damper includes a mounting fame secured to the interior of the duct which has a circular opening for air flow therethrough when the damper is open. A circular damper plate is provided which is dimensioned to block the circular opening in the closed position while in the fully open position the plate is rotated approximately 90 degrees where it is parallel to the flow direction through the duct. The movement of the damper plate is controlled by an actuator mechanism having a pair of spaced guide plates each of which is provided with two diverging arcuate guide slots. Corresponding guide slots in the two plates are aligned at the same vertical height. A movable support bracket is disposed between the two plates and supports two guide pins which extend horizontally through the aligned slots.