Two stocks of the protozoan parasite Babesia gibsoni, one of the causative agents of canine piroplasmosis, were propagated continuously in dog erythrocytes in microaerophilous stationary-phase culture. Cultures of both stocks were initiated in a humidified 5% CO2, 2% O2, 93% N2 atmosphere at 37 degrees C at a time when very few parasites (<0.01%) were detected in a thin blood smear. Cultures of one stock were also initiated in a humidified atmosphere of 5% CO2 in air at 37 degrees C during a patent parasitaemia (2.6%) in the donor animal. The culture medium was a modified HL-1 medium supplemented with dog serum, L-glutamine and antibiotics. Culture-derived parasites were cryopreserved and resuscitated. Cultures of each stock were propagated for 102 days and 51 days, respectively, before they were terminated.