ABSTRACT: Among several marine antagonistic bacteria isolated from the sea water of Kagoshima Bay, Pseudoalteromonas sp. A1-J11 was found to produce anti-Vibrio substances. The anti-Vibrio substances were extracted from the culture supernatant of the strain with chloroform and isolated using reverse-phase (Cosmosil 75C18-OPN) column chromatography followed by high-pressure liquid chromatography (Mightysil RP-18 GP Aqua). Purified substances, designated as AVS-03a, c and d showed similar ultraviolet absorption spectra with λmax at 215, 235, 315 and 327 nm in methanol. AVS-03d, the major anti-Vibrio substance, was thermostable up to 100°C and pH stable over a pH range higher than 4.0, and also showed strong inhibitory activities, specifically against Vibrio harveyi strains.