The proper use of species names depends entirely on verifying whether newly analysed specimens are conspecific with the type material. True lemmings (Lemmus) are the most common rodents of the Arctic tundra in the Old and New World and play an important role in the Arctic ecosystem; however, their taxonomy is far from resolved. The Amur lemming (L. amurensis) is the least studied and most enigmatic species of the genus. Its taxonomic position, distribution and nomenclature are uncertain due to a lack of cytogenetic, molecular phylogenetic and hybridization studies. Assignment of all true lemmings from the vast territory of western Beringia to this species has never properly been confirmed. Moreover, the type locality for this species was flooded by a newly created reservoir in 1974, making additional topotypes unavailable. In this context, genetic analysis of museum specimens, especially type material, has great potential. Here we report partial cytochrome b sequences extracted from all specimens identified as L. amurensis stored in the two largest mammal collections in Russian museums, including the holotype of L. amurensis and the type material of all forms currently considered as synonyms of L. amurensis. Phylogenetic analyses suggest that the range of the Amur lemming is dramatically smaller than previously assumed and is limited to the territories of Transbaikalia, South Yakutia and Amur in the eastern Palaearctic. Lemmus taxa from other territories, including L. amurensis ognevi and L. amurensis flavescens, refer to other lemming species. Our results impinge on nomenclature issues, taxonomy, divergence times and the evolutionary history of lemmings in the eastern part of the Palearctic and on the species concept as applied to the genus Lemmus. [ABSTRACT FROM AUTHOR]