Oocysts of Isospora species are characterized by having two sporocysts each containing four sporozoites. The sporocyst may or may not have a Stieda body. Life cycle studies indicate that species with a Stieda body are generally monoxenous and confined to the intestines, whereas those species that lack a Stieda body often use paratenic hosts, may have latent stages in the host, and may be facultatively heteroxenous. All important (valid?) species of Isospora that infect man, nonhuman primates, dogs, cats, and domesticated mammals lack a Stieda body in their sporocysts. Generic names of Levinia 1 and Cystoisospora 2 have been proposed for the Isospora species which utilize paratenic hosts but these generic names have not gained widespread acceptance.