A case of mycobacteriosis in European sea bass (Dicentrarchus labrax L.) cultured in Turkey is described. Diseased fish were characterized by lethargy, lingering passively at the water surface, pale gills, and exophthalmia. Internally, grayish-white nodules were observed in the liver, spleen, and kidney. In one individual, a granulomatous mass of renal origin protruded into the abdominal cavity. The principal histological feature was the occurrence of characteristic granulomas with associated acid-fast, rod-shaped bacteria. This is the first confirmed case of fish mycobacteriosis in Turkey.