Hepatitis B virus DNA replicates via its own polymerase that also acts as reverse transcriptase (Summers and Mason, 1982). This enzyme is encoded by a 3.5 Kb mRNA transcript covering the whole genome. Since the same transcript also codes for the core protein, and since the core open reading frame (ORF) is located upstream of the pol ORF, it has been suggested that the polymerase is first produced as a core-pol fusion protein that subsequently undergoes cleavage. This is already known to be the case with retrovirus reverse transcriptase, for which a gag-pol fusion protein is made first and the latter protein is liberated by proteolytic cleavage. We investigated this problem using mutants that were modified at the translation initiation codon for the core and precore ORF. Our findings suggested that polymerase translation occurred from the internal AUG codon independently of core protein synthesis, and that obligatory production of the core-pol fusion protein is accordingly unlikely.