Eight non-splenectomized Macaca mulatta were inoculated intravenously with strains of Babesia microti that originally were isolated from 2 human cases of babesiosis and then were maintained in hamsters in the laboratory. Patent infections developed in 7 animals with peak parasitemias of 496 to 3,906 organisms/mm3 blood. Prepatent periods ranged from 15 to 46 days. Parasitemia persisted for at least 90 days in all animals and in one, organisms were still present 559 days after inoculation.