Abstract: The ultrastructure of tissue stages of Cryptosporidium baileyi parasitising the bursa Fabricii of Shiqiza chickens experimentally infected with parasites isolated from Cherry Valley ducks was studied by transmission electron microscopy. All the stages of the protozoon excepting the motile merozoites, sporozoites or microgametes were in parasitophorous vacuoles formed by microvilli but not in contact with the cytoplasm. Close contact of the parasite with the membrane of the epithelial cell was made by means of the parasite's adhesion zone and feeder organelle between the parasite and epithelial cell. The developed schizonts had eight or four merozoites and the sporulated oocysts had four sporozoites. The anterior end of the merozoite consisted of an apical vesicle, preconoidal ring and conoid. The anterior portion of the merozoite contained rhoptries and micronemes while the posterior portion contained dense bodies, endoplasmic reticulum, golgi bodies and nucleus. No mi-cothondria, polysaccharide granules, micropore or microtubules were found. The macrogametes had 1-4 maturation bodies and two types of wall-forming bodies, as well as polysaccharide granules, endoplasmic reticulum and lipid body.