Abstract: Oocysts of C.meleagridis from quail were orally inoculated into Kunming mice and Gushi chickens to study endogenous development and pathogenicity respectively.The comparison of ultrastructure in endogenous development stages of C.meleagridis between chickens and mice was studied by scanning and transmission electron microscope.The parasitic site of C.meleagridis in mice was different from that in chickens. Cryptosporidia was detected in duodenum of mice but in ileum of chickens by scanning electron microscope.The pathogenicity of the C.meleagridis to mice and chickens is different too. Cryptosporidia was embedded deeply in microvillus of duodenum of mice, and microvilli were damaged slightly by the parasite,while cryptosporidia seemed to attach to mucous membrane of ileum in chickens,and microvilli desquamated seriously.It was concluded that C.meleagridis caused more serious damage in chickens than that in mice. Trophozoite,schizont,macrogamont,and sporulating oocyst were observed in mice and chickens by transmission electron microscope respectively,and the rough endoplasmic reticula were rich in trophozoite and schizont,particularly in middle development stage of schizogony. The depressed niche that parasites resided can be observed in the boundary between cryptosporidia and intestinal mucosa in mice by transmission electron microscope, but can’t be found in chickens.The denser and higher microvilli were discovered around the parasite than the others in mice by electron microscope.The reasons for the difference above between mice and chickens need further research. These should be new biologic characteristic of C.meleagridis.