Clinical coccidiosis in swine is associated with Isospora suis infection of piglets. The endogenous life cycle of I suis in piglets occurs within the superficial epithelium of the small intestine. Extraintestinal stages of I suis were not demonstrated in tissues of experimentally infected piglets or mice. Feeding these tissues to previously uninfected piglets did not result in oocyst shedding of consequence. Extraintestinal stages do not seem to occur in piglets with I suis as in some other Isospora species.