A total of 468 adult Dermacentor reticulatus ticks (298 females and 170 males) collected by flagging in the forests of Lubelskie province, eastern Poland, were analyzed by PCR and nested-PCR for the presence of Babesia microti DNA. In all, 21 ticks (4.5%) were found to be infected with B. microti. The infection rate in females (5.0%) was slightly greater than in males (3.5%). Detection of B. microti for the first time in adult D. reticulatus ticks suggests that this species should be considered as a potential vector of B. microti in Europe.