Cryptosporidium oocysts were isolated from stools of infected patients diagnosed as cryptosporidiosis. Suckling Swiss albino mice were infected orally by these oocysts in a dose of 10(6) oocysts/mouse. Mice were divided into 9 groups according to the duration of infection 2, 3, 4, 5, 6, 7, 10, 10, 15, 21 days postinfection, corresponding non infected control groups were included. Stools of infected mice were examined twice daily to determine; infection rate, prepatent period and intensity of infection. Mice were sacrificed according to the schedule and the whole intestine was isolated and processed for histopathological examination by H & E and modified Ziehl Nielsen stain, and histochemical examination for acid and alkaline phosphatase activity. The results revealed that the infection rate was 90% and prepatent period 3-5 days and the course of infection was 21 days. The maximum pathological changes were in the ileum. These changes were reversible.