Piroplasmosis was demonstrated in two dogs in Osaka City. Epizootiological examination revealed that these dogs had been infected with this disease in the Ikoma Mountains (Osaka and Nara Prefectures), confirming the presence of this disease in these mountains. Experiments on infection were carried out with etiological agents of this disease originated from these mountains and the Rokko Mountains (Hyogo Prefecture). In them, comparison was made on the morphology of the etiological agent and hematological findings, including red and white cell counts, hemoglobin, volume and diameter of red cells, reticulocyte count, polychromatophilic cell count, and differentia white cell count, between the two origins of the disease. As a result, it was concluded that the etiological agent of one origin was identical with that of the other origin. It was highly possible that these etiological protozoa might be identical with those of piroplasmosis which had occurred in Oita Prefecture, since there was a close resemblance in their morphology and the hematological picture they had induced. It was assumed that the present cases of canine piroplasmosis had been caused by Babesia gibsoni.