Oocysts of cryptosporidia whose morphology and measurements corresponded with those of the species Cryptosporidium baileyi were found in the stool of an immunodeficient patient. In autopsy material, cryptosporidia were found in the esophagus, whole intestine, trachea, larynx, lungs, and gall and urinary bladders. None of the 69 suckling mice inoculated with this isolate developed cryptosporidial infection. Infection was successful in 26 chickens inoculated perorally and 6 others subjected to intratracheal inoculation; it was localized in the duodenum, jejunum, bursa Fabricii, and respiratory tract. From day 12 after infection, oocysts of cryptosporidia were found in the excrement. Human infection with C. baileyi may have occurred due to disruption of the immune system by human immunodeficiency virus (HIV) and to immunosuppressive therapy undertaken after allogeneic kidney transplantation.