The development of Isospora belli, a human coccidian parasite was studied in different cell lines. Merogony was observed in all kind of cells whereas sporogony was demonstrated only in Hct-8. It was implied that not only human cell line could be infected but also some kind of animal cell lines. The unizoites could be found in vero cell. The merozoites were transferred to a new culture cell for a few passages and maintained for two weeks but the porduction of oocysts has not been observed in any culture cells during the cultivation. The development of parasites was classified into 3 categories; in specific site and specifec host, in non-specific host. From the observation, the development of parasites was different among 3 groups. Sulfamethoxazole plus trimetroprim was tested and the abnormal morphology of merzoites was observed under light microscope after H&E staining.