Photosensitivity following injection with oxytetracycline was evaluated for channel catfish (Ictaturus punctatus). In a controlled experiment, only oxytetracycline-injected fish that were exposed to direct sunlight exhibited necrotic ocular and dorsal epidermal lesions. All of the affected fish died within 8 d. All of the fish from three control groups had no symptoms. Control groups were (1) saline-injected, exposed to sunlight; (2) saline-injected, exposed to indoor light, and (3) oxytetracycline-injected, exposed to indoor light. The saline-injected, sunlight-exposed controls (all of which remained asymptomatic) were held in the pond with the oxytetracycline-injected, sunlight-exposed experimentals (all of which developed characteristic photosensitivity lesions and subsequently died). The necrotic lesions are presumed to have been caused by photosensitivity reactions when tissues exposed to oxytetracycline were also exposed to ultraviolet light.