Of 21 calves 1 to 6 weeks old inoculated with 100,000 to 750,000 Eimeria auburnensis oocysts, 19 became infected and discharged oocysts for 2 to 7 days, beginning 18 days after inoculation, with the peak number occurring most frequently 19 days after inoculation. A mild diarrhea accompanied the discharge of oocysts in several calves, and severe diarrhea occurred only in one, indicating that E. auburnensis has a relatively low degree of pathogenicity. In 5 calves killed 18 or 19 days after inoculation, catarrhal enteritis and eosinophilia were observed in the small intestine; gametocytes and oocysts were found in meso-dermal cells in the lamina propria of the villi in the lower 2/3 to 3/4 of the small intestine. The mature microgametocytes were relatively large (average, 85 by 65 u) and each contained thousands of micro -gametes, Numerous motile microgametes were observed in fresh smears. Early E. auburnensis gametocytes were found in calves killed 15 days after inoculation with E. bovis and E. auburnensis oocysts, and intermediate stages of gametocytes were seen in calves killed 16 to 17 days after being so inoculated.