Outbreaks of water-borne fungal infections of fish, amphibians, and reptiles are a common problem, especially in fish farms and hatcheries. Of particular concern is saprolegniasis, which is an infectious fungal disease that is widespread in all stages of the life cycle of fish. The disease causes serious losses in fish farms and hatcheries, especially among salmonid species. The initial clinical sign was characterized by the growth of cotton-like mycelia on the body surface, especially the head, adipose fin, and caudal fin, but the mycelia were not visible to the naked eye in the internal organs. The overall aims of this study are to gain a clearer understanding of the incidence and prevalence of saprolegniasis especially among salmonid fishes in Japan. Therefore, the author attempted to work on the objectives to investigate the Saprolegnia species associated with fungal infection in salmonids, with the goal of finding additional taxonomic criteria for such pathogenic species. Moreover, special efforts were done to develop devices for control of such infection either by using antifungal agents as substitute for malachite green and formalin or by biocontrol technique.