The genomic RNA of the potato virus X (PVX) strain HB, isolated in Bolivia and able to overcome all known resistance genes, has been cloned and sequenced. The PVXHB RNA sequence is 6432 nucleotides long and contains, similarly to the RNAs of other PVX strains, five open reading frames encoding proteins of M(r)s 165.1K, 24.5K, 12.4K, 7.6K and 25.1K (coat protein), respectively. Multiple amino acid sequence alignments of the coat proteins of four PVX strains identified eight amino acid residues unique for PVXHB. Structural prediction comparisons of the coat proteins of PVXHB and of the other strains suggest a general structural similarity. However, two of the eight amino acid residues unique for strain HB gave rise to a change in the predicted coat protein structure, suggesting a possible involvement in the resistance-breaking activity of PVXHB.