Attempts were made to find eicosapentaenoic acid (EPA) -producing microorganisms. Out of some 7, 000 bacterial strains screened, 112 of them showed more or less this activity. Most of them were isolated from intestinal contents of marine fishes which were known to contain high levels of EPA in their body lipid. A bacterial strain isolated from the Pacific mackerel Pneurnatophorus japonicus showed a high productivity of EPA (3.8 to 10.8mg/g dry cells) and also a high ratio of EPA to total fatty acids (27.8 to 36.3%). The strain was Gram-negative, motile, obligate aerobic and short rod-shaped, which we judge it to be a new species close to genus Alteromonas.