Topotypes of Microtus guentheri were compared with specimens of the genus Microtus with 2n=54 chromosomes from western Anatolia. According to morphological and biometrical comparisons, it was determined that M. guentheri is a different species from Microtus lydius. The dorsal colour and the shape of the baculum distinguish M. guentheri from M. lydius orphologically. In addition, ear length, interorbital constriction, mastoid breadth, height of tympanic bulla and weight of M. guentheri were found to be statistically different from those of the nominate subspecies of M. lydius (p<0.05). In this respect, a new subspecies of M. lydius is first described from central Anatolia: Microtus lydius ankaraensis ssp.n. In addition, our findings revealed that the population known as M. guentheri in Europe needs taxonomic revaluation.