Woodchuck hepatitis virus (WHV) was discovered in serum samples from captive woodchucks (Marmota monax) at the Penrose Research Laboratory in December, 1977. WHV belongs to the same class of viruses as hepatitis B virus (HBV), the cause of serum hepatitis in man. Both appear to be associated with chronic hepatitis and hepatocellular carcinoma in their respective hosts. Woodchucks were trapped and blood samples collected to determine the prevalence of WHV in natural woodchuck populations. Sera from 217 woodchucks trapped from southeastern Pennsylvania, central New Jersey, and north central Maryland during the spring and summer of 1978 and 1979 were tested for evidence of WHV infection. In 1978, 7 of 51 (13.7%) woodchucks were positive for WHV antigens and in 1979, 28 of 166(16.9%) tested positive. In addition, 49 of 166 (29.5%) woodchucks trapped in 1979 had antibodies to WHV antigens. The data indicate a high prevalence of WHV in woodchucks from the areas surveyed.