Most of our knowledge of the structure and function of the hepatitis B virus (HBV) genome comes from the analysis of the viral DNA sequences cloned in bacteria1–3. Because the physical state of cloned HBV DNA differs from HBV DNA encapsidated in the virion—for example, it lacks the nick-gap structure4, and a covalently linked protein5—the question arises as to whether it can initiate HBV replication in vitro6 or in vivo. We describe here the development of typical acute viral hepatitis, and the detection of HBV-specific DNA sequences in the serum and liver, in a chimpanzee inoculated with cloned HBV DNA. This demonstrates that neither the virion proteins nor the nick-gap structure of the virion DNA are needed for the initiation of replication of HBV in vivo.