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The closely related species Ixodes persulcatus and I. pavlovskyi are vectors of agents tick-born encephalitis and
Lyme disease. These species have two great disjunctive sympatric distribution areas (fig. 1), where both species inhabit the same biotopes, and during the same season. In these conditions there are prerequisites of contacts of opposite sexes of the different species during ambushing the hosts on vegetation. Sequence of "switching on" of the morphological, physiological, and hostparasite factors of reproductive isolation has been established. The features and the range of manifestation of these factors at different combinations of sexes and species of the partners have been revealed. The studied factors, such as precopulative and postcopulative components of the mechanism of reproductive isolation of I. persulcatus and I. pavlovskyi were considered. A hypothesis for sympatric origin of the species as a result of specialization the adults of I. persulcatus to mammals, and I. pavlovskyi to birds is proposed.