Six species of the genus Hydrachna parasitizing beetles were found by investigating 35,150 water beetles (six families and 244 species). Hydrachna geographica, H. inermis, and H. incognita were the most numerous and were collected from 22 species of Dytiscidae and Hydrophillidae. The infestation rate varied from 0.1 to 71.4%. The water mite parasitized several species but preferred some of them. Larvae were usually found under the elytra, but the specimens of H. inermis were attached on the surface of the body. The size of larvae varied from 0.15 to 5 mm. The relations between the size of larvae and the sampling suggested that the reproduction occurs from spring to summer. The investigated species of water mites chiefly colonize spring astatic waters where no typical coleopterofauna has been determined. This fact suggests that the zone of infection is separated from the zone of parasitism.