None of the mutations so far discovered in several hepatitis delta virus (HDV) isolates appears to determine important changes in HDV specific protein (HDAg) expression, except for a putative mutation at nucleotide 1012 converting an amber stop codon (TAG) to a codon for tryptophan (TGG). Here we present the characterization of an HDV obtained from the liver of a woodchuck inoculated with sera from fulminant HDV patients in Central African Republic (CAR). By restriction enzyme analysis and sequencing of HDAg-coding region cDNA clones, we found that this HDV isolate bears a novel mutation (T to A) at nucleotide 1013 which converts the amber stop codon (TAG) to a codon for lysine (AAG). Comparison of these nucleotide sequences with those available from American, Japanese, Taiwanese, French, Italian and Nauru isolates showed a variability of 1.7 to 21.5% and 1.9 to 28.7% at the nucleic acid and amino acid levels, respectively. The HDAg-encoding sequence of the CAR isolate is closely related to that of the Italian HDV isolate. The in vitro expression of this HDV isolate resulted in a unique HDAg species (28K) which was identical with that characterized in vivo.