Abstract <p>Using scanning electron microscopy, a comparative morphological analysis of the fine structure of the guard hairs in the mummy of an adult male ancient wolf (<i>Canis lupus</i> ssp.) at an age of 45 504 ± 150 BP, found in the late Pleistocene deposits in Yakutia, was carried out. Guard hairs selected for maximum thickness (guard I) vary significantly in different areas of the wolf’s skin in profile and metric data, and the vibrissae show a specific medullary structure. The fine structure of the cellular medulla of the hair in the ancient wolf is shown to be similar to that of extant representatives of Caninae, such as the jackal (<i>C. aureus</i>), maned (<i>Chrysocyon brachyurus</i>) and red (<i>Cuon alpinus</i>) wolves, some breeds of domestic dogs (<i>Canis familiaris</i>) (Akita Inu, Basset, Dalmatian, Irish Terrier, Miniature Schnauzer, Russian Greyhound, and Hortaya Greyhound), and African fox (<i>Vulpes pallida</i>). In these species, the disordered (laced or spongy) medulla of the thickest part of the hair shaft (“shield”) differs from the ordered (cellular, with regularly located large cavities) medulla of the red (<i>V.</i> <i>vulpes</i>), gray (<i>V. ferrilata</i>), and Tibetan (<i>Urocyon cinereoargenteus</i>) foxes, arctic fox (<i>Alopex lagopus</i>), corsac fox (<i>Vulpes corsac</i>), and American corsac fox (<i>V. velox</i>). By this feature, it is possible to distinguish the hair of wolves and foxes, if we compare the medial part of the hair, since at the base of the hair a cellular medulla is also found in some breeds of dogs (Akita Inu, Mastino Napoletano, Miniature Schnauzer, and Hortai Greyhound) and corsac fox. The cuticle pattern varies greatly along the guard hair shaft, which is typical of wolves and foxes. This is especially specific in the area of the shaft in front of its thickened part, as it consists of lanceolate or diamond petal scales extended along the hair. The cuticle pattern formed by lanceolate or diamond petal scales is quite similar in ancient and extant wolves, arctic foxes, and African foxes, as well as some domestic dog breeds (Akita Inu and Basset). However, the cuticle of the hair of many other species differs from that of wolves. Thus, a feature such as the presence of lanceolate or diamond petal cuticle on the hair is of limited importance for distinguishing between the species. We have shown that the hair of an ancient wolf in its fine structure (the thickness of the shaft, the degree of development of the medulla and its microanatomy, and the cuticle pattern and its variability along the guard hair shaft) practically does no differ from that of extant wolves, thus confirming that the wolf as a species had already been fully formed in the Pleistocene. The fine structure of wolf hair differs from that of foxes, but has features similar to the hair of the jackal and domestic dog, which confirms the phylogenetic relationship with the last two genera.</p>