The aim of this study was to determine the prevalence of spirochete Borrelia burgdorferi s.l. in ticks Ixodes ricinus and mosquitoes Aedes spp. within the Bukowa Forest, collected between 2000 and 2001. The study covered 215 ticks (193 nymphs and 22 adults) and 947 mosquitoes female of the genus Aedes. Spirochetes of Borrelia burgdorferi s.l. were detected in the arthropods studied with the method of indirect immunofluorescence assay (IFA). Positive readings of the immunological reaction were stated in 17.7 % of the collected nymphs and adult forms of Ixodes ricinus, and in 0.8 % of mosquito females of the genus Aedes. The number of B. burgdorferi observed in a view field (400 x) of microscopic preparations of all infected mosquitoes and about 10 % of the infected ticks, ranged from 1-10. This number in 50 % of the nymphs was from 11-50 spirochetes. View fields of the preparations of the other 50 % of nymphs and adult forms featured more then 50 spirochetes. The observed low values of the prevalence and infection intensity of female mosquitoes Aedes spp. compared to ticks suggest that the former do not pose a serious epidemiological threat in the spreading of Lyme disease.