Trypanosoma equiperdum can be cultivated in the developing chick egg by inoculation of the infected material into the yolk sac of the egg. An inoculum of 2 million trypanosomes per egg gave satisfactory results. In 93% of infected eggs microscopically visible infection occurred, or infection could be demonstrated by mouse passage. When trypanosomes could not be found microscopically in the egg, mice inoculated with the egg specimen usually became infected. Sometimes it took 14 to 35 days to have such mice die with T. equiperdum infection. Trypanosomes which did not infect the embryos seemed to have been destroyed or retained in the yolk sac. Death of infected embryos was accompanied with hemorrhages in the brain, liver, and heart. Inocula of 500,000 and one million trypanosomes caused infection in the egg. With inocula of 150,000 or less organisms satisfactory results were not obtained. Repeated passages in the egg do not change the virulence of T. equiperdum. To maintain strains in eggs it is necessary to inoculate a sufficient dose, i. e., 2 million organisms, to use several eggs, and to transfer once a week. The advantages of the above method are: 1. The injection into the yolk sac is a simpler method than injection on the chorio-allantoic membrane. 2. By injection into the allantoic cavity the embryo is killed in (some) 5 days and frequent transfers are necessary. The injection into the yolk sac retards the process of infection in the embryo.