Wild duck populations were investigated over a 4 year period for duck hepatitis B virus (DHBV) infection and liver disease. It appeared that DHBV is endemic in wild migratory mallards from France and the U.S.A., although neither hepatocellular carcinoma nor viral DNA integration could be detected in liver samples examined. The follow up of natural infection indicated that wild mallards developed significantly higher serum titres to DHBV DNA than Pekin ducks. The results of experimental transmission demonstrated that such differences in viraemia were not related to the breed of ducks but to the virus isolate, since the wild mallard-isolated DHBV (DHBV WM) induced significantly higher viraemia in both mallard and Pekin ducklings compared to the domestic Pekin DHBV (DHBVP) isolate. The naturally infected mallard and Pekin ducks had only minor histological lesions of the liver compared with experimentally infected birds. There was no correlation between the intensity of viraemia and the severity of liver lesions, suggesting that as for mammalian hepadnaviruses the hepatic injury in DHBV-infected ducks is probably immunologically mediated.