In the summer of 1977, a local epizootic occurred among wild and cultured populations of ayu in Shiga Prefecture. A vibrio was isolated from diseased fish showing petechie on the body surface and congestion of the internal organs. This vibrio was confirmed to be the causative agent of the disease by artificial infection in healthy ayu. The isolate was examined for DNA base composition and DNA homology as well as morphological, physiological, biochemical and serological characteristics. Biochemically and physiologicaly, the organism was very similar to Vibrio cholerae but it did not show positive agglutinating reaction against anti-V. choterae sera (Ogawa and Inaba). The percent guanine plus cytosine content was 47.0-47.7. The DNA homology value between the isolate and V. cholerae NIH35A3 was 86%. On the basis of the results obtained, the organism was classified as non-cholera vibrio.