Thirty-five cases of Babesia gibsoni infection and 11 cases of Babesia canis infection were diagnosed and treated in dogs at Kadena Air Base in Okinawa, Japan, between April 1979 and February 1980. Diagnosis was made by demonstrating the organisms in blood smears and by serologic examination, using an indirect fluorescent antibody test. Serologic findings correlated well with the occurrence of infection in mature dogs, but poorly in young (1- to 3-month-old) dogs. Although these 2 intraerythrocytic parasites were readily distinguishable on Giemsa-stained blood smears and by the indirect fluorescent antibody test, the clinical syndromes were similar. Most dogs were anorectic and depressed and were found to have regenerative anemia. Of 37 dogs tested, 31 were Coomb's test-positive, and most of these became Coombs' test-negative after treatment and disappearance of clinical signs. Specific treatment consisted predominantly of the use of diminazene aceturate. Pentamidine isethionate also used. Although these drugs were effective in halting and reversing the clinical progression of the disease, they usually were ineffective in clearing the blood of B gibsoni organisms, and relapses commonly occurred. Both drugs appeared to be more effective against B canis. It was concluded that some of the several hundred dogs arriving in the United States annually from Okinawa are carriers of B gibsoni, a parasite only recently discovered in North America.