The purpose of the studies was to determine the prevalence of Borrelia burgdorferi sensu lato in selected populations of Ixodes ricinus in Finland and to secure strains of the spirochete for further characterization. 1,210 Ixodes ricinus ticks (399 females, 419 males and 392 nymphs) were collected during June to August 1992 by flagging from 8 sites in 3 regions. The frequency of B. burgdorferi infection was determined by isolation in BSK II medium. The species identity of most of the isolates was confirmed with the immunofluorescence method. 67 B. burgdorferi strains were isolated from 8 sites; the prevalence ranged from 2.8% to 7.9%. The overall isolation percentage for adult male ticks was 6.4% (7.5% for females, 5.3% for males); for nymphs, 3.8%. No statistically significant association of the prevalence was observed with either pasture or off-pasture habitats nor with specific geographic region/regions. Prevalence figures were roughly of the same magnitude in areas and parts of the country known to differ in their incidence of human borreliosis. An additional 294 ticks, mainly engorged females, were collected from places outside the main study sites. Of the 7 positive ticks 3 were engorged females, originating from a cow, a dog and a cat, respectively. The results in general demonstrate that tick populations in various parts of Finland quite commonly harbour B. burgdorferi.