Streptococcal infections have been frequently observed in cultured freshwater fish, tilapia (Tilapia nilotica), rainbow trout (Salmo gairdneri) and ayu (Plecoglossus altivelis) at farms in various districts of Japan. The causative agents isolated from diseased tilapia, rainbow trout and ayu had the same morphological as well as biochemical characteristics. All the strains were also serological homogeneous. These strains were found to be pathogenic to freshwater fish after intraperitoneal injection. The autoclaved and hot-HCl treated cells did not react with any of the group specific sera used: Lancefield A, B, C, D, E, F, G, H, K, L, M, N, O and MG. This organism was not identical to any strains of Streptcoccus previously reported.