We karyotyped six species of Microtus voles collected along the southern edge of their range in northern and western Iran. Diploid and fundamental numbers were as follows: M. socialis and M. paradoxus 2n = 62, FNa = 60,
M. qazvinensis 2n = 54, FNa = 54, M. transcaspicus 2n = 52, FNa = 52, and
M. mystacinus (= M. rossiaemeridionalis) 2n = 54, FNa = 54. Two cytotypes were retrieved in M. irani from its type locality: 2n = 48, FNa = 46 and 2n = 64, FNa = 62. While our results confirmed an early report of 2n = 64 for this vole, the 2n = 48 cytotype remains unexplained. Karyological variability is relatively low in social voles and chromosomal data contribute little to individual species recognition. We argue that Arvicola mystacinus
De Filippi, 1865, described from Lar Valley (north-east of Tehran) is the oldest available name for 2n = 54 voles with the following synonyms:
M. subarvalis Meyer, Orlov & Skholl, 1969, M. epiroticus Ondrias, 1966, and
M. rossiaemeridionalis Ognev, 1924.