The russet mite, Aceria anthocoptes (Nal.), is the only eriophyid that has been recorded on Canada thistle, Cirsium arvense (L.) Scop. It has been noted in several European countries and recently in the USA. With its apparent host specificity and because of the damage it causes to its host plant, A. anthocoptes is being studied as a potential candidate for classical biological control. The aim of the present study was to examine quantitative morphological traits in four populations of A. anthocoptes living on two infraspecific host plant taxa (C. arvense var. arvense and C. arvense var. vestitum) in two geographically separate areas of Serbia in order to test the hypothesis of absence of the possible host plant impact on mite morphology. MANOVA analysis revealed significant differences between populations from different localities in Serbia. Populations of A. anthocoptes inhabiting two thistle varieties in the vicinity of Belgrade differed significantly from mites inhabiting the same two host varieties in the vicinity of the town of Ivanjica. Canonical discriminant analysis showed that the trait which best discriminates the populations of A. anthocoptes is the number of dorsal annuli. It was not possible to ascribe morphological differences to the impact of the host plant.