Abstract: Fifty Hy-Line chicks were divided into 5 groups 10 for each.They were orally inoculated,8.0×106,1.6×106,3.2×106,6.4×106 oocysts of Cryptosporidium baileyi respecitivelly when they were 2 days old.The chick were killed at different intermission time postinfection,got trachea,burea of Fabricious (BF),and throat for study by scanning electron microscopy.C.baileyi endogenous development occurred primarily in the trachea,BF and throat.There are two kind of schizont (type Ⅰ meronts contained eight merozoites,types Ⅱ meronts contained four merozoites).The freed sporozoites or merozoites attached to epithelial cells where they become drumstick-like from banana-like,and then formed spherical trophozoites.There were two kinds of parasitophorous vacuoles (one had spherical residual body in it and the other had no such thing).Microgamete escaping from maternal body and endogenous development stages in goblet cell were observed.Cryptosporidia caused cilia lodging arranging disorder,deciliaing and cilia fusing of chicken trachea,epithelium membrane of bursa swollen with microvillus deciliaing or fusing,arranging disorder,excess mucus secretory product,mononuclear cell infiltration.Rugae were formed on the epithelium membrane because of epithelial cell hypertrophy or hyperlasia.