The sequences of cDNA clones encoding most of the NIb protein, the coat protein and the 3' untranslated region of papaya ringspot virus (PRV) strains W and P have been determined. The open reading frame of P strain PRV was confirmed by amino acid analysis. Nucleotide sequence comparisons of these strains show that they share a 98.2% identity in their NIb gene regions and a 97.7% identity in their coat protein genes. The sequences of these two strains are distinct from other potyvirus types, confirming their classification as two strains of the same virus. The NIb amino acid sequence possesses conserved amino acids characteristic of RNA-dependent RNA polymerases. Comparison of the coat protein amino acid sequence with those of other potyviruses shows perfectly conserved amino acids which may have functional significance.