The present paper characterizes a natural focus of tick-borne borreliosis in the Yaroslavl Region, analyzes the incidence of this infection in the Region, and thus concludes that the density of the population living in the areas of the natural foci of tick-borne borreliosis has a significant impact on its epidemic process. The incidence of tick-borne borreliosis in the Yaroslavl Region has remained a challenge in the past 7 years. By the number of cases, this infection heads the list of naturally focal infections recorded in the Region. Examining long-term changes in the incidence and prevalence of tick-borne borreliosis in the Region revealed its high incidence in the population residing in the tidal marshlands and forest areas. At the same time the potential activity of natural foci of borreliosis due to the conditions that maintain the population of Ixodes ticks is high in the forest-field, southern, and meadow-forest-field regions.