Histopathological observations were carried out on 17 domestic cats naturally affected with feline panleukopenia. Principal lesions were found in the intestine, bone marrow, and lymphoid organs. Intestinal lesions were characterized by degenerative changes accompanied by the appearance of intranuclear inclusion bodies in the epithelial cells of the crypts. In contrast to the crypts, the villi were seldom involved. Hypoplasia, parenchymal degeneration, and activation of the reticuloendothelial system were observed in the bone marrow and lymphoid organs. Intranuclear inclusion bodies were found occasionally also in the reticular and parenchymal cells of the bone marrow, lymphoid organs, liver, adrenals, and pancreas. Most of the inclusion bodies were amphophilic when stained with hematoxylin and eosin and occupied the whole area of the nucleus without producing any zone of clear halo. While cells bearing inclusion bodies underwent degenerative changes constantly in the intestinal crypts, the formation of inclusion body was not accompanied by the degeneration of corresponding cells in any other organ. Pathological changes as mentioned above were considered to be closely related to the systemic infection of feline panleukopenia virus.