Lettuce mosaic potyvirus (LMV) causes severe disease of commercial lettuce crops. LMV isolates show wide biological variability, particularly in their ability to overcome the resistance genes described in Lactuca sativa. For a better understanding of the molecular interaction between lettuce and LMV, biological and molecular characterization of a collection of 10 LMV isolates known to differ in virulence or aggressiveness was performed. The ability of these isolates to overcome the resistance genes was reevaluated under standardized conditions. To study the molecular variability of LMV, an immunocapture-reverse transcription-poly-merase chain reaction technique, coupled with direct sequencing, was used to obtain nucleotide sequence data from three short regions of the LMV genome. Clustering analysis was performed and compared to the biological properties of the 10 isolates. Three groups of LMV isolates were discriminated based on the molecular data. These groups appear to correlate with the geographic origin of the isolates rather than with their pathogenicity. Sequence comparison with California isolates clearly showed that the California isolates are related to the western European isolates, raising the possibility of past exchanges of LMV between western Europe and California.