Most procedures that have been described for purifying Cryptosporidium parvum oocysts are designed to either identify the parasites in clinical specimens or isolate oocysts from a small volume of feces from infected animals. The present study describes a rapid method for purifying high numbers of C. parvum oocysts from feces of infected calves that contains minimal contaminating fecal material and bacteria. The isolation method is based on differential flotation of C. parvum oocysts in NaCl, followed by ether extraction to solubilize lipids in calf feces. This procedure regularly yields >10⁹ purified C. parvum oocysts within 1–2 days of feces collection.