The association of morphological structures and the degree of covariation in males and females of Dermacentor marginatus (Sulzer) (Acari: Ixodidae) are measured in specimens collected from its distribution area, using the Principal Axis Factor Analysis method applied to several morphological peculiarities. The leg structures are scarcely correlated one with other, with the exception of the spur on trochanters II and III in the males. There are not significant partial nor multiple correlations in the covariation of dorsal scutal pattern. A very high degree of variation in gnathosomal and spiracular features has been observed, and these structures are strongly correlated between them; however the allometric growth of palpi, basis capituli, and cornua, is not linked. None of these anatomical peculiarities are useful in the taxonomie separation of the marginatus complex of ’species’, although the paratypic series of D. niveus was included into the studied specimens. The cladistic analysis display the existence of several ’groups’ of specimens, or ’morphotypes’ , and a clinal variation can be observed in the length and breadth of some gnathosomal structures; such a variation is not evident in the shape of spiracular plate nor relative size of dorsal palpal spur.