A total of 320 of 621 (52%) captive Atlantic cod, Gadus morhua, died from fin rot over an 8-year period; mortality reached high proportions in 1979 when 110 of 168 (66%) succumbed. Lesions were confined to the fins, skin and dermal musculature and were observed 3-10 days after the fish were placed in laboratory aquaria. Erosion of the fins and caudal peduncle, accompanied by petechiae and ulceration also were apparent in the trunk region. Most deaths occurred within 2 months after capture. Infection was associated with depressed hematocrit, hemoglobin and total plasma protein and an increase of circulating immature erythrocytes and neutrophils. Mortality was probably due to physiological stress resulting from excessive blood loss. Three genera of bacteria, mostly Pseudomonas, but also Aeromonas and Vibrio, were isolated from fin rot tissues and possibly are the causative agents of the disease.