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RAINBOW TROUT (S•lmo galrdner11) from three localities in western Washington, submitted to us for examination during the last 3 years, have shown a characteristic disease. The pathology is produced by an organlsm apparently hitherto unrecognized in this area or in fresh-water fish in this country. An eplzootlc of serious proportions developed in one of the areas and is reported in this paper.The agent that produces the disease is a spherical organism l0 to 80 microns in diameter occurring throughout the viscera, especially the kidney, liver, and spleen. We have assigned it to the g•nus Ic.hth•vos•orldlum Caullery and Mesnll, 1905 (sy•. Ichth•ophonus Plehn and Mulsow, 1911). During the last 50 years, organisms of this general group in European marine fish have been reported by about a dozen authors. There have been about the same number of reports from American marine fish and somewhat fewer from European fresh-water fish. The only reference to Ichthyosporldlum in freshwater fish in North America is that of Fantham and Porter (1948), who described organisms quite different from those we have seen. Sproston (1944) gives an adequate review of the literature. To our knowledge, the most recent paper is that of Scattergood (1948). Robertson's flgures (1909) of a disease in a small flounder and a haddock are typical of the parasite of the rainbow trout with which we are concerned.to 40 by 100 feet. All were supplied by a common water source arising from springs in and around a brush-filled pond. The drainage through the fish-pond area involved many interconnections between ponds, so that very few had independent, previously unused water supplies.The owner, who had purchased the fish farm 6 years before, had previously experienced no major eplzootlc of any kind. He had developed his own brood stock; so there was no introduction of fish from other areas.Two points of possible significance should be considered. In order to carry heavier loads of fish in the ponds, a large pump was installed in the lowest or last pond of the series and water was pumped to the head ponds for recirculation. Also, the viscera from fish cleaned for market were fed to the younger fish.The losses, which were extreme in all ponds, varied. Although no definite records were kept, the owner estimated the losses on a percentage basis.The main loss was composed of the 2-inch fingerlings which were about 3 months old and the 6-inch fish which were slightly.more than a year old. The losses in these groups amounted to about 90 percent. It is interesting to note that the larger fish in each pond seemed to become infected and die first. Later in the season, the 7-to 8-inch, l•-year-old marketable fish and the brood stock which were about 3 or more years old suffered a mortality amounting to about 50 percent. In all, losses were estimated at 50,000 fish. DIAGNOSIS The infection in rainbow trout was brought to our attention in the spring of 1952, when excessive mortalities became apparent and the owner of a private fish farm sought aid. The farm contained abo...

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