The life-cycle of Isospora ohioensis in dogs is described. Eight dogs were killed 6, 16, 24, 48, 72, 96, 114 and 120 h after feeding 106I. ohioensis sporocysts to each. First divisional stages occurred in the jejunum at 48 h post-inoculation (p.i.). At 48 h, zoites occurred in pairs in parasitophorous vacuoles of surface epithelial cells of the jejunum. The parasitophorous vacuoles were 7–9 × 6 μm and the zoites were 7–9 × 2·5 μm in sections. At 72 h, uninucleated zoites, multinucleated zoites and meronts containing fully formed merozoites occurred in surface epithelial cells of the jejunum. At 96, 114 and 120 h asexual multiplication occurred throughout the small and large intestine, mostly in the ileum. The number of asexual generations was not determined. At least 2 structurally distinct meronts were identified at 96–120 h p.i. Type I meronts contained larger merozoites (11 × 3 μm) than those (7·5 × 1·5 μm) in Type II meronts. Meronts were merozoite-shaped and contained up to 8 nuclei. Uninucleated, binucleated and multinucleated zoites occurred within the same parasitophorous vacuole. Gamonts occurred in surface epithelial cells of the small intestine, caecum and colon, but predominantly in the ileum, 96–120 h p.i. Macrogamonts were 13–17 × 11–12 (14·5 × 12·8) μm in sections and 21–26 × 17–25 (21·7 × 17·6) μm in smears. Microgamonts were 13–17 × 8–15 (15·3 × 11·4) μm in sections and 24–30 × 15–24 (27 × 19) μm in smears and contained up to 50 microgametes. The cycle was also studied in 6 dogs killed 6, 12, 24, 48, 96 and 120 h after feeding tissues of laboratory mice that had been infected with I. ohioensis oocysts 17 days earlier. Stages were not seen in the dogs killed at 6 and 12 h p.i. Asexual and sexual stages were structurally similar in size and location to the oocyst-induced cycle but their development was faster by 24 h in the mouse-induced cycle. Extra-intestinal stages were not found in sections of mesenteric lymph nodes, spleen, lung, liver, heart, skeletal muscle and brain. However, biological evidence indicated that I. ohioensis invaded the spleens and mesenteric lymph nodes of dogs fed oocysts.