The nucleotide sequence of tomato ringspot nepovirus (TomRSV) RNA1 has been determined. TomRSV RNA1 is 8214 nucleotides in length, excluding the 3' poly(A) tail, and contains a single long open reading frame (ORF) of 6591 nucleotides beginning at the first AUG codon at nucleotide position 78. This ORF accounts for 80% of the RNA1 sequence and would give rise to a polyprotein with a predicted molecular mass of 244 kDa. Amino acid sequence comparisons between portions of the TomRSV RNA1-encoded polyprotein and proteins encoded by several members of the picornavirus superfamily have provided information concerning the genomic organization and putative functions of TomRSV-encoded proteins. The putative TomRSV protease retains a conserved histidine residue present in the proteases encoded by members of the como-, poty- and poliovirus groups which is thought to be involved in dipeptide cleavage site recognition. Interestingly, this histidine residue is replaced by a leucine in the proteases of other sequenced nepoviruses. This suggests that the TomRSV protease shares dipeptide cleavage site specificity with that of como-, poty- and picornaviruses rather than the other nepoviruses.