An epizootic of fungus disease occurred among kokanee salmon, Oncorhynchus nerka var. adonis in Lake Shikotsu of Hokkaido Island in 1974 to 1975. Fungi infected mainly on the peduncle and caudal fin of fish. One kind of fungus was isolated from diseased fish by inoculating materials from the caudal fin onto Sabouraud dextrose agar and incubating at 15°C. Inoculation experiments using moon fish, Xyphorus maculatus showed that all the isolates were virulent (Table 2).The fungus belonged to the genus Saprolegnia according to DICK (1973) from its characteristics of the asexual reproductive stages. Characteristics of the sexual reproductive structures of diclinous antheridia and subcentric oospores placed the fungus close to S. parasitica and S. australis according to SEYMOUR (1970). Comparison in biological characteristics between the present fungus, and S. parasitica and S. australis described by SEYMOUR (1970) are summarized as follows.The zoospores of the fungus were larger in size than those of S. parasitica and S. australis. The oogonia were sparsely formed only after prolonged period of time, and in this point it resembles to S. parasitica rather than S. australis. The fungus can be distinguished from S. parasitica and S. australis by the characteristic of catenulate oogonia. The fungus has smaller oogonia than those of S. parasitica and S. australis. As regards characteristics of pits of the oogonial wall the fungus lies between S. parasitica and S. australis. Oospores per oogonium of the fungus were less in number than those of S. parasitica and S. australis. Antheridial barnches are usually present in S. parasitica and S. australis, but were often lacking in the fungus.From the above mentioned the fungus was regarded as a new species of the Saprolegnia.