(Uploaded by Plazi for the Bat Literature Project) Background: In Europe two ixodid bat tick species, Ixodes vespertilionis and I. simplex were hitherto known to occur. Methods: Bat ticks were collected from cave walls and bats in Hungary. Their morphology and genotypes were compared with microscopy and conventional PCR (followed by sequencing), respectively. Results: A year-round activity of I. vespertilionis was observed. Molecular analysis of the cytochrome oxidase subunit I (COI) gene of twenty ticks from different caves showed that the occurrence of the most common genotype was associated with the caves close to each other. A few specimens of a morphologically different tick variant were also found and their COI analysis revealed only 86-88% sequence homology with I. simplex and I. vespertilionis, respectively. Conclusions: The microenvironment of caves (well separated from each other) appears to support the existence of allopatric I. vespertilionis COI genotypes, most likely related to the distance between caves and to bat migration over-bridging certain caves. The name I. ariadnae sp. nov. is given to the new tick species described here for the first time.