Infection with Sarcocystis (Protozoa: Sarcocystidae) was diagnosed in 130 of 153 (85%) samples of muscle from mule deer around Bozeman, Montana. Three structurally distinct mature and microscopic sarcocysts with characteristic cyst walls were found. Cyst walls of type I sarcocysts were about 2 μm thick and had characteristic inverted tee-shaped villar projections; these cysts were considered to be S. hemionilatrantis Hudkins and Kistner, 1976. Cyst walls in type II sarcocysts were thick-walled (about 7 μm) and their villar projections were 6.7 × 1.1 μm. The core of the villar projections consisted of granular material and some filamentous structures. Bradyzoites were 11.6 × 2.8 μm and were tightly packed in compartments. Cyst walls of type III sarcocysts were also thick-walled (about 9 μm) but the villar projections were 8.5 × 4.7 μm. Bradyzoites were 13 × 3.3 μm and were loosely arranged in compartments.