Abstract: The ultrastructure of Cryptosporidium parvum, which parasitized on the surface of the intestine of the immunosuppressed mice,was studied by transmission electron microscopy. The parasite was enveloped by the parasitophorous vacuole developed from the host epithelial cell. An electron dense band was found at the attachment site between the Cryptoporidium and the host cell. At the attachment site,the cytoplasma membrane of Cryptosporidium appeared in the form of many membranous folds,and contacted closely with the plasma membrane of the epithelial cell. Rhoptry-like structure and small electron dense granules were found in the merozoite,but their effects on the development of Cryptosporidium ar?still unknown (Figs. 1 - 5).