Cystoisospora belli, formerly known as Isospora belli, is a coccidian parasite that causes a diarrheal illness like Cyclospora cayetanensis and Cryptosporidium species. Its oocysts are elliptical and relatively large, measuring 25 to 30 micrometers in length. The oocysts are immature when excreted and contain only a single sporoblast. In the environment, they mature to contain 2 sporoblasts. Cystoisospora belli oocysts can be easily seen on wet preparations and also stain deep pink/red using modified acid fast stains (like the other coccidia).