The gametogonous and sporogonous stages of Sarcocystis fusiformis were studied in the small intestine of 10 cats each fed 350 macroscopic sarcocysts and killed 12 h, 3, 5, 7, 9, 11, 13, 15 and 17 days post infection. The macrogamonts were observed from 12 h up to 9 days post infection, whereas microgamonts were seen only on Day 3 and were very few in number. Developing oocysts were seen from Day 5 to Day 11 post infection, and from Day 13 fully sporulated oocysts and sporocysts were found in increasing numbers.