Peroral and intraperitoneal inoculation of severe combined immunodeficient (SCID) mice with cystozoites of three coccidia of the genus Sarcocystis (Protozoa, Apicomplexa; S. dispersa, Sarcocystis sp., and S. muris) revealed that after peroral administration, only S. muris could develop in the immunodeficient mouse host. The cystozoites of S. muris transformed into gamonts and, after fertilization, performed sporulation with the production of infectious sporocysts in the small intestine of the SCID mice. Impaired immunity is probably responsible for the unusual behavior of S. muris (which is normally the heteroxenous mouse-cat parasite) in the SCID mice. We hypothesize that the phylogenetic distance between the intermediate and final hosts is the reason why cystozoites of the two other Sarcocystis species tested (S. dispersa with a mouse-owl cycle and Sarcocystis sp. with a murine rodent-snake cycle) could not develop when inoculated into SCID mice.