Oocysts of Isospora canis, an important protozoan parasite of dogs, must sporulate in the external environment in order to become infective. To determine the tolerance of these oocysts to different temperatures, oocysts ware collected from freshly passed feces, cleaned, and subsequently stored in unsporulated, partially sporulated, and completely sporulated conditions at temperatures ranging from +50 C to -50 C. Observations on survival for each group were determined periodically thereafter at intervals up to 64 days. The rate of sporulation was followed at 6 hr intervals for temperatures of 20, 25, 30, and 38 C. Completely sporulated oocysts ware more resistant to temperature extremes than unsporulated or partially sporulated oocysts; the optimum storage temperature for all stages was 4 C. This suggests that the development and survival of this parasite would be higher during mild spring and fall months than during cold winter and hot summer periods. The sporulation time for I. canis was 102 hr at 20 C, 96 hr at 25 C, 60 hr at 30 C, and 54 hr at 38 C; however, at the upper limit tested, more abnormal development occurred.