In 1994, 4204 out of 4656 Ixodes ricinus collected both from vegetation in three different areas and from hunter-killed deer and wild boar in the Białystok province (eastern Poland) were examined individually for the presence of Borrelia burgdorferi s.I. Detection of spirochetes was carried out by the routine indirect immunofluorescence assay (IFA) using polyclonal antibody (anti-B. burgdorferi, strain 1 B 29). B. burgdorferi spirochetes were evident in 349 (8.8%) nymphal and adult I. ricinus collected from vegetation (n = 3958) and in 8 (3.2%) of those removed from hosts (n = 246). Among the ticks collected from vegetation, infection rates in nymphs (5.8-6.4%) in particular areas were about 2-4 times lower than in adults (8.1-24.6%). The calculated minimal and maximal infection rates of ticks collected from different sites were 1.6% and 15.4%, respectively. Prevalence of B. burgdorferi in I. ricinus was determined with respect to the abundance and seasonal activity of the ticks.