The course of parasitaemia of three strains of Trypanosoma avium Danilewsky, 1885 were followed in the avian host. Indications of a cyclical increase in circulating parasites was obtained. Evidence also indicated that the level of parasitaemia was correlated with the number of flagellates initially inoculated and that previous infection with T. avium conferred a degree of immunity on the avian host.