Sporogony of Isospora suis of swine is described. Unsporulated oocysts were collected immediately after defecation, mixed in 2.5% potassium dichromate solution, incubated at various temperatures, examined at different time intervals, and classified as to stage of development. The uninucleate sporont underwent nuclear division and became binucleated, and then divided to form two round, uninucleate sporoblasts. Binucleate sporoblasts elongated to form binucleate sporocysts, each of which then gave rise to four sporozoites. Sporulation was completed within 56 hr at 20 C, 40 hr at 25 C, 16 hr at 30 C, and 12 hr at 37 C. No sporulation occurred at 4, 40, 42, 45, or 50 C. The thermal death point was between 37 and 40 C.