The infectivity ofCryptosporidium muris (strain RN 66), originally isolated from the house rat (Iseki 1986), to various laboratory animals was studied by transmission experiments. After oral inoculation with 1×106 oocysts, mice, guinea pigs, rabbits, dogs, and cats all discharged endogenously produced oocysts in their feces. Among these host species, mice and cats were highly susceptible to the parasite. The prepatent period for six 3-week-old specific pathogen-free (SPF) mice was 5 days postinoculation (PI), the patent periods varied between 34 and 75 days for each mouse, and the number of oocysts discharged per individual per day (OPD) was 11–46×106 at the maximum on days 16–26 PI. The total number of oocysts discharged per mouse during the patent period was estimated to be 170–560×106. Three inoculated cats (1–2 months old) also discharged a large number of oocysts for a long period. Guinea pigs, rabbits, and dogs showed low susceptibility to this strain; the OPD was extremely small and the patent periods were less than 3 weeks. The entire endogenous development of this parasite occurred in the stomach and not in the small and large intestines of these experimental animals. Because of this lack of host specificity, it is suspected thatC. muris could be infective to humans, especially immunocompromised patients such as those with AIDS.