Skin ulcer disease of flounder Platichthys flesus (L.) was investigated at 7 sampling sites in the northern Baltic Sea, SW Finland, during 1987-1989. Disease prevalences ranged from 1.8 to 11.1% in different areas. A significant correlation was established between sex, fish length and disease prevalence. Males were more affected (10.7%) than females (3.1%) and larger fish showed higher disease prevalences than smaller ones. The ulcers were not randomly distributed on the fish trunk. They occurred mainly on the blind side of the males and on the eye side of the females. Ulcers on males tended to occur close to the lateral line; on females, the ulcers were scattered over the whole trunk. Bacterial examination of diseased specimens resulted in the isolation of a cytochrome oxidase-negative, atypical Aeromonas salmonicida, primarily from the ulcers.