Dissemination, season variability, ways of infection, as well as main carriers of Lyme's disease and tick-borne encephalitis, reasons for the population contact with natural foci and ticks, age and socially professional status were shown to be extremely similar in patients with borreliosis and encephalitis in Leningrad Province. This fact indicates principal similarity of basic epidemiological properties in two etiologically different infections. A much higher morbidity level is characteristic of Lyme's disease. This is associated with differences in the level of carrier infectivity with Borrelia or viruses. The possibility of mixed infection with two agents in endemic regions is stressed and the practical value of the problem is emphasized.