The coat protein gene nucleotide sequences from eight previously uncharacterized strains of potato virus X (PVX) were determined. Comparison of the deduced amino acid sequences showed that two classes of PVX coat protein, designated types X and B, could be distinguished based on protein length and overall amino acid identities. In all there were 14 amino acid positions where all of the type X proteins differed from all of the type B proteins. The PVX coat protein is the principal viral determinant of the outcome of interactions between the virus and potatoes carrying either the Nx or Rx1 resistance genes. The different strains of PVX were tested for their ability to overcome resistance conferred by three potato resistance genes: Nx, Nb and Rx1. All of the strains that were avirulent on potato cultivars carrying the Nx resistance gene were found to have type X coat proteins whereas strains capable of overcoming the Nx resistance had type B coat proteins.