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Thymus, an aromatic medicinal plant, is a well-known, perennial and woody herb from Lamiaceae family. Thymus is taxonomically a very complex genus with a high frequency of hybridization and introgression among sympatric species, and some species of this herb are endemic to Iran. In the present study, in order to identification genetic variability in this medicinal plant seven Thymus spp. accessions collected from different regions of Iran along with London agricultural species were studied by karyotypic characteristics. The secondary basic numbers in all ecotypes was x = 15 that probably originate from a primary basic number x = 7. The ploidy levels of these ecotypes were diploid and tetraploid. The Thymus ecotypes under study occupied classes 1A and 1B of Stebbins’ karyotype classification, indicating the presence of a primitive symmetrical karyotype in these ecotypes. The mean chromosome length ranged from 1.03 to 1.53 µm. chromosome types were detected as metacentric “m”. Furthermore, the cluster analysis using chromosomal parameters and based on UPGMA assigned the ecotypes into four groups.