A 2-yr-old female brook trout, Salvelinus fontinalis, maintained at a state fish hatchery in New York (USA) was culled from the breeding stock. At necropsy, it had a markedly enlarged kidney. On histopathological examination, lymphoblastic lymphoma was diagnosed in several organs including kidney, dorsal musculature, intestines, liver, gill, pseudobranch and ovary. Origin of the tumor was uncertain, although renal involvement could have been the primary site of development.