One hundred and fourteen strains of Vibrio and allied genera, including 9 strains from sea water, 58 strains from normal marine fish and 28 strains from diseased fish and shell-fish were submitted to morphological, cultural and biochemical examinations. The results were scroed as 190 two-states features, and phenotypical similarity between strains was calculated adopting the simple matching coefficient of SOKAL and MICHENER. The mutual relationships among the strains were analysed by hierarchical clusterings on the similarity values and by examining the homogeneity of populations by the method of ROGERS and TANIMOTO. Both analyses revealed two large, distinct phenons, phenon 1 and 2, both of which could be regarded as species. Phenon I comprised most strains from diseased fish and shell-fish, besides standard strains of V. parahaemolyticus and V. alginolyticus. Their reactions on sucrose fermenta-tion and VP tests were various. Phenon 2 was composed mostly of strains from intestines, gills and luminous organs of normal fish. This phenon corresponded to Photobacterium phosphoreum that was recently redescribed by HENDRIE et al., but less than a fifth of the strains of the phenon were luminescent.