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Fish gills function as both respiratory and excretory organs. Basically they consist of a network of capillaries where blood is separated from the surrounding water by only one or two layers of cells. Proliferation of epithelial tissue, and later the loss of surface by the clubbing and fusing of lamellae, impair respiration and the excretion of nitrogenous waste materials, and disturb osmotic balance. Because these changes adversely affect the health of fish, the prevention and treatment of gill diseases are important in fish culture. Pathologic changes in gill tissues have been divided into five categories: (1) bacterial gill disease (BGD) of salmonids and pondfishes, caused by filamentous bacteria that have yellow pigment in their cell walls; (2) nutritional gill disease, caused by a deficiency of pantothenic acid; (3) hemorrhagic gill disease, in which an apparent hemorrhagic condition is really a dilation of lamellar vessels (telangiectasia), caused by toxic agents or physical injury; (4) mycotic gill necrosis in pondfishes, caused by the fungus Branchiomyces (Meyer and Robinson 1973; Niesh and Hughes 1980); and (5) proliferative gill disease of unknown etiology in pondfishes and rainbow trout, Oncorhynchus mykiss (Daoust and Ferguson 1985). Bacterial gill disease, the most lethal of these categories, is discussed here.