Cryptosporidium parvum oocysts isolated from calf feces were examined by scanning electron microscopy during excystation. Intact C. parvum oocysts were spheroid to ellipsoid, approximately equal to 3.5 X 4.0 micron, with length : width ratio = 1.17. The oocyst wall had a single suture at one pole, which spanned 1/3 to 1/2 the circumference of the oocyst. During excystation the suture dissolved, resulting in a slit-like opening, which the sporozoites used to exit the oocyst. Sporozoites were 3.8 X 0.6 micron and had a rough outer surface.