In-frame and frameshift mutations were introduced into the pre-S region (1,212 base pairs) of duck hepatitis B virus. The in-frame mutants retained the inserted 12 nucleotides, while the frameshift mutants either reverted to wild type or exhibited a 10-nucleotide compensatory deletion downstream of the original mutation site. Thus, although duck hepatitis B virus has a compact and highly economical genome organization, it can replicate despite alterations of up to 9 amino acid codons in the pre-S and P open reading frames.