This thesis describes an investigation of the presence and importance of 'Isospora suis' in Ontario suckling and weanling pigs. A cross-sectional study of suckling piglets on 50 conveniently sampled herds found 'I. suis' oocysts on 70% of the farms and an association between diarrhea and the presence of oocysts. Risk factors for a positive farm were small herd size and not using detergent as part of the cleaning process in farrowing room sanitation. A longitudinal study was performed on 3 farms and showed that pigs from litters shedding oocysts in their feces at 2 or 3 weeks of age grew slower (>1 kg lighter at 8 weeks of age) than pigs from negative litters. The overall conclusion from these studies was that isosporosis is a common and economically important disease of pigs in Ontario.