Isospora suis had 3 asexual and 1 sexual intra-intestinal conventional life cycle. The first asexual generation was most prominent at 2 days p.i. (post inoculation) and produced 2-7 merozoites. The second-generation meronts were prevalent at 3-4 days p.i. and produced 2-12 large merozoites. At 4-5 days p.i. the third generation meronts were prominent and produced 4-24 small crescent shaped merozoites. Mature sexual stages were most prominent at 5-6 days p.i. The stages were most numerous in the distal half of the small intestine. At 8-9 days p.i. stages morphologically similar to the second generation of meronts reappeared, followed by the further development into third generation merozoites and sexual stages. This was reflected in a prepatent period of 5 days and a biphasic patent period of 5-8 or 9, and 11-14 days p.i. Intraperitoneal injection of liver/spleen and intestinal lymph node homogenates, respectively, from piglets infected 24 and 48 h, previously with high doses of oocysts, resulted in a patent infection 10-12 days post inoculation of the donor piglets. No differences in the life cycle of I. suis were observed between conventionalized and germ-free piglets. An extra-intestinal life cycle of I. suis related to the second patent period was postulated.