The effect of diminazene aceturate on splenectomized and nonspienectomized dogs with Babesia gibsoni (B. gibsoni) infection was investigated. In splenectomized dogs, the fissional and multiplicational stages of B. gibsoni were observed in peripheral blood films, and hemoglobinuria was frequently observed. These findings were different from previous reports and were not changed by administration of diminazene aceturate. It is clear that the intramuscular administration of diminazene aceturate at the dose of 3 mg/kg body weight for 3 days is not effective against B. gibsoni infection in splenectomized dogs.