Over a 12-month period, four white-tailed deer (Odocoileus virginianus) were encountered in Michigan which had multiple cysts of the liver. No cystic lesions were present in other organs of the carcasses of three of the animals available for study. Light and electron microscopic observations on the lesions showed that they resembled a polycystic deformity of the biliary system, similar to that described previously in man. Polycystic liver must be differentiated from hepatic echinococcosis, which it resembles closely in gross appearance.