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The recently shown phenomenon of natural hybridization between Ixodes persulcatusand Ixodes pavlovskyiticks (Kovalev et al., 2015) stimulated similar studies in the sympatric zones of other tick species. In the present paper, 265 Ixodes ricinusand I. persulcatusticks from Estonia were subjected to a search for interspecific hybrids based on nuclear (ITS2) and mitochondrial (cox1) markers as well as morphological features. Surprisingly, only 72.1% of ticks morphologically identified as I. ricinusactually were I. ricinusboth at nuclear and mitochondrial markers, while the accuracy of morphological species identification for I. persulcatuswas 99.3%. Among ticks morphologically identified as I. ricinus, 24.6% turned out to be interspecific hybrids and 3.3% were I. persulcatus. Generally, about 11% of the individuals studied were shown to be interspecific hybrids with different levels of nuclear DNA introgression. The analysis of hybrid populations proved the mating pair female I. ricinus×male I. persulcatusto form hybrids more efficiently, then female I. persulcatus×male I. ricinus. The same trend can be observed for backcrosses preferentially mating with I. ricinus. Hybridization between I. ricinusand I. persulcatusproved the existing view about their reproductive isolation to be untenable. Interspecific hybridization occurring between both closely (I. persulcatusand I. pavlovskyi) and more distantly (I. ricinusand I. persulcatus) related Ixodesspecies could introduce novel alleles that modify vector competence, host use or the ability to exploit diverse microhabitats.