A 3-year-old child with severe acute self-limited enterocolitis was found on rectal biopsy to be infected with the protozoal parasite Cryptosporidium. This organism is known to infect a variety of vertebrates, but this is the first report of infection by Cryptosporidium in a human being. Both light and electron microscopic findings in the rectal biopsy are reported. It is suggested, on the basis of the severity of the clinical symptoms, and on the pathological changes in the rectum, that the organism in this case is likely to have been the cause of the enterocolitis and thus to have been a pathogen rather than a commensal. The source of the infection in this child could not be established. The value of signoidoscopy and biopsies is noted in this condition and as a general method for determining the etiology of a gastrointestinal infection in cases where other studies are negative.