Worldwide, at least 14 viruses have been isolated from fish and 10 or more are known from electron microscopy. The known effects of the isolated viruses range from virulent diseases to benign infection. Six viruses cause significant mortality in fish husbandry and 3 of them are enzootic in North America. A herpesvirus is responsible for an acute hemorrhagic disease and high mortality in fry and fingerling channel catfish at temperatures of 25°C and higher. In the Pacific North West and Alaska a rhabdovirus produces an acute viscerotrophic response in rainbow trout, Chinook and sockeye salmon, but only at temperatures of 12°C or lower. Infectious pancreatic necrosis virus is a reovirus-like agent that infects salmonids - at times killing young trout, at times not. Key features of each disease will be discussed, and characteristics including pathological changes will be illustrated. A 7-minute color film on infectious pancreatic necrosis and virological examination of fish will complete the presentation.