To investigate the genetic diversity of potato virus M (PVM; genus Carlavirus, family Betaflexiviridae), the complete nucleotide sequence of the coat protein gene of 30 PVM isolates from a major potato-growing region in Iran were determined. Phylogenetic analysis of these Iranian PVM isolates together with those available in the GenBank database suggested two divergent evolutionary lineages that did not reflect the origin of the isolates, and these were designated as PVM-o and PVM-d. Examination of the genetic variability of the coat protein of Iranian isolates and their counterparts whose sequences are available in the Genbank database revealed 16 genotype groups in the PVM population. Analysis of the synonymous-tononsynonymous ratio showed strong purifying selection in the CP gene in the genotype groups of divergent clades.