Dermacentor marginatus (Sulzer 1776), a distinctive tick of the Palearctic faunal region is redescribed from 941 specimens collected throughout its geographical range. The species display a very high intraspecific morphological variation, especially at the level of palpal dorsal spur, size of cornua, spiracular plate, scutal pattern, and female genital aperture, features commonly used in the determination of several species in the subgenus Serdjukovia Santos Dias. However, the relative proportions of the basis capituli and palpi are almost invariable even in specimens from separate geographical zones. Several species and subspecies have been added to the subgenus, on the basis of slight variation in the above-mentioned structures, without comparison to appropriate reference material. Bearing in mind such a variation, it is concluded that D. niveus Neumann and D. daghestanicus Olenev are synonyms of D. marginatus.