In an exptl. study of the life history of E. bovis, a sporozoan parasite of cattle, a total of 51 calves was used. These were inoculated per os with mature oocysts, or directly into the intestine or into a Thiry''s fistula with schizonts and merozoites. Enterectomies and other surgical operations were also employed. Sporozoites were found in calves killed at intervals 15 to 46 hrs. after inoc. Observations were made on excystation both in vivo and in vitro. Schizonts were found in calves killed as early as 5 days, but were most abundant in those killed 16-18 days after inoc. They were of the Globidium type, macroscopic, producing an average of 120,000 merozoites each. Schizonts occurred in the central lacteals and possibly also in connective tissue cells of villi, chiefly in the posterior half of the small intestine. Apparently there is a single generation. Sexual changes were first seen 17 days after inoc. and were usually limited to the cecum and colon. Development of these stages is described. Discharge of oocysts began in most cases 18-20 days after inoc.