The subgroup J avian leukosis virus favors the myelocytic series cells and causes myeloid leukosis (myeloblastosis and myelocytomatosis). Natural cases of myeloblastosis (myeloblastic myeloid leukosis) are uncommon and usually occur in adult chickens. This paper describes clinical signs and gross and histopathologic features of myeloblastosis in an adult female budgerigar (Melopsittacus undulatus) that was infected naturally. At necropsy, the spleen was greatly enlarged (enlarged seven or eight times normal) while the other visceral organs were normal. Histologic examination of the spleen indicated a massive intravascular and extravascular accumulation of myeloblasts with a variable proportion of promyelocytes and myelocytes in the red pulp of the spleen.