Abstract: Object: The present study was undertaken to compare the viability and infectivity of Cryptosporidium parvum oocysts (CPO) that had been stored for 1, 4, 7, 10, 13, 16, 20, 25 and 30 months at 4°C in 2.5% potassium dichromate (K2Cr2O7), respectively. Methods: An excystation protocol was performed in vitro to evaluate the viability of oocysts. One hundred and eighty female BABL/c mice were used to evaluate the infectivity of oocysts by detecting the prepatent period of C. parvum infection, the quantity of oocysts excreted, and the number of parasites that colonized the villi of the ileum. Results: CPO preserved in K2Cr2O7 for 1–16 months were capable of excystation in vitro and infection of mice in vivo. The prepatent period was extended from 3 to 8 days as the storage time of oocysts increased. There was a strong correlation between prepatent period and duration of oocyst storage (R2＝0.92). Large number of oocysts were isolated and purified from feces of mice inoculated oocysts preserved for 1-16 months in K2Cr2O7, respectively. Cryptosporidia parasitized in the terminal ilea were found in mice inoculated with oocysts stored in K2Cr2O7 for 1-16 months. Conclusion: CPO may be stored at 4°C in K2Cr2O7 for at least 16 months for the purposes of laboratory research, and K2Cr2O7 is a good medium of keeping viability and infectivity of CPO for a long time.