Comparisons of the nucleotide sequences and amino acid sequences were made among various hepadnaviruses,and the relative evolutional distances were calculated.From the comparison,it was shown that DHBV,HBV,GSHV and WHV are derived from a common ancestor,and the three mammalian hepatitis B viruses are more homologous to each other than they are to DHBV,indicating that DHBV starts to evolve on its own earlier than the three other viruses,as do birds compared with mammals.Based on these findings it was proposed that viruses evolved in a fashion parallel to the species they infect. From the calculated relative evolutional distances, it was suggested that WHV and GSHV can be categorized in a way similar to the subgroups of HBV.