The sequence of a 5' segment of PVY RNA overlapping the 5'NTR (10 isolates) and the P1 coding region (8 isolates) was established. The data have been used to quantify the polymorphism in the 5' part of the genome. Percent nucleotide identity for the 5'NTR, percent nucleotide identity and aminoacid similarity for the P1 coding region have been established for each pair of isolates. These data concerning either the 5'NTR or the P1 allowed the clustering of isolates into three groups illustrated by phylogenetic trees. Group I comprises potato isolates all inducing "tobacco veinal necrosis" symptoms. Group II contains isolates inducing either "tobacco veinal necrosis" or mosaic symptoms in tobacco. Group III comprises mainly pepper or tomato isolates inducing mosaic symptoms in tobacco and shows a geographical clustering of the Tunisian isolates. This clustering into three groups is discussed in comparison with phylogenetic trees previously obtained from sequence analysis of the capsid gene in the PVY species.