Abstract: A thermally actuated deflector structure for automatically varying the air discharge pattern of a ceiling diffuser in response to a sensed change in the temperature of air being delivered to the diffuser includes a multi-bladed damper assembly positioned over a central outlet portion of the diffuser and having a frame portion secured to the diffuser. The blades in the damper assembly are drivable between a cooling orientation in which they block the central diffuser portion and preclude supply air flow downwardly therethrough, and a heating orientation in which they uncover the central diffuser portion and permit supply air flow downwardly therethrough. A coiled bimetallic actuating strip is positioned above the damper assembly and has an inner end anchored to the damper frame, and an outer end secured to a drive member linked to the damper blades.