Some bacterial diseases such as pseudotuberculosis (Pasteurella piscicida infection)and streptococcal infection(Streptococcus spp. infection)have been prevailing among cultured populations of yellowtail(Seriola quinqueradiata) in Japan. In the course of diagnostic works about bacterial infections of yellowtail in Tokushima Prefecture from 1976 to 1978, it was found that an additional bacterial disease, i.e. vibriosis, had been causing considerable mortalities for those three years in addition to the above-mentioned two diseases.The occurrences of the three diseases or the three pathogens in 1977 was demonstrated in a table, where it was shown that the vibriosis took place in the young fish (Body weight 3.7-42.5g) from late May to early July.The external sign of the vibriosis was characterized by opaque or congested eyes, eroded fins, darkened body color, and hemorrhagic or necrotic lesions on body surface and musculature.Vibrio sp. was isolated from these diseased fish, and submitted to characterization tests, agglutination tests with anti-V. anguillarum sera, and a pathogenicity test with healthy yellowtail.Putting all the results together., the etiological agent of the disease was identified as Vibrio anguillarum. This is the first record of V. anguillarum infection in yellowtail, though another vibriosis, the causative agent of which was reported as Vibrio sp. by KUSUDA (1965), has been known.