Circumpolar variations in the size and shape of the skull in the Arctic fox Vulpes lagopus L. 1758 was studied based on 31 linear measurements of 326 specimens. The specimens were collected from the mainland parts of North America and Eurasia, as well as from several islands within the native range of the Arctic fox, namely, Greenland, Iceland, Svalbard, St. Lawrence Island, St. Matthew Island, Pribilof Islands (St. Paul Island and St. George Island), and Commander Islands (Bering Island and Mednyi Island). A specimen from the Arctic fox populations introduced to the Atka Island and Aleutian Islands was also included in the analysis. Morphological circumpolar variations in the skull of Arctic foxes observed in the present study were compared to the genetic variations reported in earlier studies. No significant differences were observed in cranial size and shape between Arctic foxes from the mainland (both North American and Eurasian populations) and those from St. Lawrence Island, St. Matthew Island, Svalbard, and Iceland. Although the Greenland foxes were significantly smaller than the mainland ones, their cranial shape was comparable. Based on the skull shape, the Commander Islands' Arctic foxes were the most distinct among all populations. An unsupervised model-based cluster analysis implemented in MCLUST showed that, based on cranial morphology, the Commander Islands' Arctic foxes could be differentiated from the other Arctic foxes with a high level of probability (p >= 0.95). The morphological specificity of the Commander Islands' Arctic foxes is congruous with their genetic isolation. The morphological specificity of Greenland and Pribilof Island Arctic foxes was greater than their genetic distinctiveness. A taxonomic revision of the study species was performed based on the data presented. The Arctic foxes from the mainland parts of North America and Eurasia, St. Lawrence Island, St. Matthew Island, Svalbard, and Iceland were classified under a single nominotypical subspecies, V. l. lagopus L. 1758, while the Greenland Arctic foxes belong to a separate small subspecies of the mainland Arctic fox, namely, V. l. groenlandicus Bechstein 1799. Similarly, the Pribilof Arctic foxes V. l. pribilofensis Merriam 1903 are subspecies of the mainland animal and further should be divided into two separate subspecies, one from St. Paul Island and another from St. George Island, taking into account the level of their morphological differences, Based on marked morphological and genetic differences, the Commander Islands' Arctic foxes were reclassified as a distinct species, V. beringensis Merriam 1902, as proposed by Ognev (1931), with two subspecies, namely, V. b. beringensis Merriam 1902 from Bering Island and V. b. semenovi Ognev 1931 from the Mednyi Island. The specimen from Atka Island was identified to the species from the Commander Islands, suggesting that the Arctic fox population on Atka Island must have been introduced from the Commander Islands. Given the declining population of the Arctic foxes on the Commander Islands, this data is important for the environmental and genetic conservation of the Commander Islands' Arctic fox
