Dermacentor reticulatus ticks is one of the most important vectors and reservoirs of infectious microorganisms in Europe. In Poland, this species is common in areas east of the Vistula river, but increasing numbers of new localities are being reported in the west of the country. The aim of the study was to determine the estimated size of the D. reticulatus tick population in eastern Poland in different types of ecological habitats. To this order, the multiple mark-release-recapture technique was used. The largest estimated population size of D. reticulatus was noted in an agriculturally unused open meadow undergoing progressive ecological succession (344 ± 57.8 specimens/100 m2). Besides the ecological type of the habitat, the availability of hosts probably exerts the greatest effect on the tick population size in the analyzed area.