The complete nucleotide sequences of the genomic RNAs of the 0 and E isolates of lettuce mosaic potyvirus (LMV) have been determined. These two isolates differ by their behavior towards two lettuce resistance genes and by their seed transmission properties. LMV-0 is unable to induce disease in lettuce carrying either one of the mol(1) and mol(2) recessive resistance genes, whereas LMV-E is able to induce disease in the same plants. The genomes of these two isolates are 10080 nucleotides (nt) in length, excluding the poly(A) tract, and encode polyproteins of 3255 amino acids (aa). The open reading frame is flanked by a 5' non-coding region of 103 nt and a 3' non-coding region of 212 nucleotides. Ten proteins were predicted. The P3 protein. with 377 aa,is the longest potyviral P3 protein characterized to date while the P1 protein, with 437 aa, is among the longest pi proteins reported. Sequence comparisons between the two isolates demonstrated only limited sequence difference. The overall nucleotide and amino acid sequence identities between LMV-0 and LMV-E are 94 and 97% respectively. The greatest variability occurs in the P1 and in the variable N-terminal region of the coat protein, while the NIa protease domain, the NIb protein, the C-terminus of the helper component protease and the 3' non-coding region are extensively conserved. While this sequence analysis does not allow direct identification of determinants involved in the resistance breaking or in the seed transmissibility properties, these data are a first step towards the characterization of these determinants.