Cryptosporidiosis is reported in a healthy 25-year-old male. Clinical symptoms include 1 day of nausea and low-grade fever and 9 days of diarrhea, followed by 3 days of constipation. Oocysts of Cryptosporidium sp. were present in sugar flotations of the first fecal sample collected 56 hours after onset of the symptoms and in daily fecal samples collected through day 12 of the illness. Oocysts of the human isolate of Cryptosporidium sp. were morphologically indistinguishable from those obtained from naturally and experimentally infected calves. After 1 week of sporulation at room temperature, oocysts from the human and from calves contained four sporozoites and a spherical residuum. When inoculated orally, sporulated Cryptosporidium sp. oocysts of human and of calf origin produced indistinguishable infections in suckling mice and rats and in adult mice.