In 1993, 2178 out of 3816 Ixodes ricinus and 45 out of 82 Dermacentor reticulatus collected from vegetation at 35 sites and removed from hunter-killed deer in Olsztyn province (N. C. Poland) were examined individually for the presence of Borrelia spirochaetes. Detection of spirochaetes was carried out by the routine indirect immunofluorescence assay (IFA) using polyclonal antibody anti-B. burgdorferi, strain 1 B 29. Borreliae, presumably B. burgdorferi, were evident in 192 (11.5%) I. ricinus (nymphs and adults) collected from vegetation (n = 1666) and in 6 (1.2%) of those removed from hosts (n = 512). Among the first ones, infection rate in nymphs (7.5%) was 2.5 times lower than in adults (18.8%) but was similar in females (18.7%) and males (18.9%). The calculated minimal and maximal infection rates of ticks collected from different locations were 2.9% and 35.7%, respectively. No spirochetes were observed in D. reticulatus tested.