A Babesia species isolated from a captive caribou (Rangifer tarandus caribou) with clinical signs of babesiosis and a circulating parasitemia was cultured in vitro. Normal adult caribou erythrocytes supported the growth of the Babesia sp., as did erythrocytes from white-tailed deer (Odocoileus virginianus). Two basal media (M-199 and RPMI-1640) and a defined medium (HL-1) each supplemented with adult bovine serum were compared. The most favorable growth of the parasite occurred in HL-1 medium with 20% adult bovine serum. The morphology of this Babesia sp. isolate shared some characteristics with B. odocoilei and B. divergens.