Objective To isolate and identify piroplasms from a Japanese blood donor by using NOD/shi-scid mice. Methods The donor's blood specimen was inoculated into NOD/shi-scid mice, whose circulating erythrocytes(RBC) had been replaced with human RBCs(hu-RBC-SCID mice). Results Parasites proliferated within the human RBCs in the mice, resulting in a high level of parasitemia, and the parasites were related closely to %Babesia microti.% Conclusion Our results demonstrated the usefulness of hu-RBC-SCID mice for isolation of parasites from human and for maintenance of the parasite infectivity for human RBCs.