Amprolium reduced the number of oocysts shed by Eimeria acervulina, E. maxima, E. necatrix, and a mixture of susceptible strains of E. tenella. Sporulation of oocysts from mediated chickens was reduced compared with that of oocysts from unmedicated chickens. Sporulation was reduced by levels of 0.0250% amprolium for E. acervulina and by levels of 0.0060% for E. maxima and the susceptible E. tenella. Not enough oocysts were recovered to measure sporulation of E. necatrix. Sporulation reduction was not affected by the method of administration of amprolium (feed or water), except with E. acervulina, for which fewer oocysts sporulated when 0.0120% amprolium was added in the drinking water than when 0.0125% amprolium was added to the feed. Conversely, amprolium medication had no effect on the sporulation of an amprolium-resistant E. tenella. When fed to unmedicated chickens, those oocysts from amprolium-medicated chickens that did sporulate were as infective as oocysts recovered from unmedicated chickens.