Field ex-periments were conducted to study the incidence of coccidia in young Hereford calves during two grazing and feeding seasons. Oocysts of Eimeria auburnensis, E. bovis, E. canadensis, E. cylindrica, E. ellipsoidalis, E. subspherica, and E. zurnii were identified. Eimeria zurnii appeared to be the causative agent of "winter" coccidiosis although E. bovis was frequently present in small numbers. The incidence of infection, as determined by periodic fecal examination, increased from zero after birth to 100 per cent approximately 2 months after weaning. The incidence rate did not always include the more pathogenic E. zurnii and E. bovis; however, there was a marked increase in the incidence of infections caused by E. zurnii and E. bovis during the fall and winter. Tranquilizers were used in one experiment, and some calves were weaned slowly in attempts to alleviate the stress of weaning and to prevent outbreaks of "winter" coccidiosis. Nevertheless, outbreaks occurred during each of the study years.