Biliary cryptosporidiosis was studied by inoculation of 2 x 10(5) Cryptosporidium baileyi oocysts (AU-B1 isolate) into the gall bladders of ten 6-day-old broiler chickens. Clinical signs of disease were not seen. Three of the 10 chickens developed biliary tract infections, based on histologic examination of tissue sections. Lesions seen in the gall bladders of these birds included epithelial hyperplasia and infiltration of the underlying connective tissue with mononuclear leukocytes. One of these birds also had involvement of the hepatic bile ducts. The bile ducts were mildly dilated and contained lesions similar to those seen in the gall bladder. Few to many cryptosporidia were present in the gall bladders and bile ducts of infected birds. Chickens may be of use in the study of biliary cryptosporidiosis, a common sequel to enteric infection in humans with human immunodeficiency virus infection.