To determine the infectivity of Cryptosporidium to hosts in slight infections, we examined the infectivity and oocyst output patterns of Cryptosporidium muris in mice inoculated with small numbers of oocysts. One of the 25 ICR mice inoculated with 2.4 × 10¹ oocysts and 19 of the 25 mice inoculated with 2.4 × 10² oocysts shed oocysts in the feces after inoculation. Four of the 50 mice inoculated with 2.4 × 10¹ oocysts for 10 consecutive days also shed oocysts and their OPG values were similar to that of the mice which received 2.4 × 10 ² oocysts. Consequently, it is clear that less than 10% of the mice which received 2.4 × 10¹ C. muris oocysts for 10 consecutive days.