The life-cycle of canine Isospora bigemina was studied in dogs, cattle, cats and mice. Under experimental conditions dogs served as both definitive and intermediate hosts. Unsporulated oocysts (11 X 12 mum) were shed in the faeces and sporulated outside the host within 12 h at 30-37 degrees C and 36 to 48 h at 23 degrees C. Sporulated oocysts measured 12 X 13 mum and contained 2 sporocysts which in turn contained 4 sporozoites. Sporocysts averaged 6 X 9 mum and sporozoites averaged 2 X 6 mum. Although no stages were found microscopically in tissues or in faeces of 23 dogs orally inoculated with sporulated oocysts, the oocysts were infectious because 5 of 14 dogs that ingested tissues of dogs fed oocysts, shed oocysts. Prepatent periods were 7 to 15 days; patency was 1-3 days. No dogs became ill. In the naturally infected dog and one experimentally infected dog, schizonts were found in epithelial cells, distal to the host cell nucleus at the tips of villi throughout the small intestine. Schizonts were 5-7 mum and contained 3-12 merozoites. In the naturally infected dog, gametes were found in the same location. Male gametocytes were 6-8 mum and contained 6-12 microgametes, and female gametes were 7-8 mum. Sporulated I. bigemina oocysts from a naturally infected dog were not infectious to cattle, cats or mice. Structurally identical oocysts were shed by 2 dogs after ingesting hearts and diaphragm from naturally infected cattle; these oocysts were also not infectious to cattle. Although dogs acted as both intermediate and definitive hosts under experimentation, this is not likely to occur in nature. A canine-bovine-canine cycle would appear to be the natural mode of infection but I. bigemina was not infectious to cattle under experimental conditions.