F URUNCULOSIS, or as it has been more appropriately termed, septicemia, is a disease primarily affecting salmon and trout. It is caused by the invasion and growth of Bacterium salmonicida Emmerich and Weibel, a Gram negative, non-spore forming, diplobacterium belonging to the family Bacteriaceae Cohn. After gaining entrance to the host, presumably by way of the digestive tract, the organism is spread by the blood stream and produces focal necrosis and subsequent liquefaction throughout the tissues. The more conspicuous gross lesions are those of the body musculature, characterized by the formation of deep seated boils or bloody blotches-blisters filled with liquefied muscle tissue and blood. Under favorable conditions, the muscle lesions enlarge rapidly and eventually rupture through the skin producing a characteristic, ragged, deep, undermining type of ulcer. Although the muscle lesions are most conspicuous, essentially the same progressive necrosis and liquefaction are to be found throughout the internal organs, particularly in the spleen and kidneys. The host has no adequate defense mechanism against this disease and no verified recovery from furunculosis has ever been recorded. Cases may be arrested by low water temperatures or other adverse factors, only to break out with renewed vigor when conditions again become favorable. The reader is referred to Plehn, Davis, Williamson, and Duff and Stewart for a more complete description of furunculosis. Furunculosis was originally described from hatchery fish in Germany by Emmerich and Weibel (1894). Following their description, the disease was apparently commonly recognized in German trout hatcheries and it was believed to be a strictly hatchery disease. Plehn dispelled such a belief in 1909 by finding furunculosis among wild fish in the province of Bavaria. Extensive studies by this author in 1909 and 1910 revealed the disease to be present in wild trout taken from 25 rivers and streams of that province. Shortly thereafter, furunculosis was found in other parts of Germany and in Switzerland, Austria, and France as well. The disease was first observed in Great Britian among mature salmon taken from 4 rivers of southwestern England in 1911 by Masterman and Arkwright. Three years later, it was found among salmon in Ireland by Mettam. Studies by the British Furunculosis Committee, appointed in 1929 to investigate the disease, revealed furunculosis to be present in many watersheds of the British Isles. The first definite proof that furunculosis existed in the western hemisphere was offered by Marsh, 1902, who found it to be the cause of an epizootic among hatchery fish in Michigan. It has since been found at many trout and salmon hatcheries throughout the United States. Furunculosis was recently described from wild Rocky Mountain whitefish, Dolly Varden, and cutthroat trout of the Elk River in southeastern British Columbia by Duff and Stewart, 1933.