Lyme borreliosis, the most important vector-borne disease in the Northern hemisphere, causes health problem for populations in endemic areas. In the present study, the density of questing Ixodes ricinus ticks and their infection with Borrelia burgdorferi sensu lato (sl) was examined in 11 areas located on the Swiss Plateau and in an alpine valley. From 1999 to 2001, free-living I. ricinus ticks were collected on a monthly basis by flagging vegetation in these areas. Each tick was examined for the presence of B. burgdorferi sl using direct fluorescent antibody assay, and for isolation of the bacteria. Borreliae were characterized by PCR followed by RFLP. Density of questing ticks varied greatly between studied areas. Borreliae were observed in ticks collected in all investigated sites. However, the prevalence of infection differed significantly among areas. Infection prevalence varied from 9% to 40% in nymphs and from 22% to 47% in adults. Adult ticks were significantly more infected (129/366, 35%) than nymphs (109/552, 20%). There was no correlation between nymphal density and infection prevalence as well as between adult density and infection prevalence, but there was a correlation between density of ticks and density of infected ticks. During the spring peak of questing tick density, a range of 2-30.3 infected ticks per 100 m(2) was observed. B. burgdorferi sl isolates (n = 129) were obtained from ticks collected in 10/11 areas. Five Borrelia species were identified: B. garinii, B. burgdorferi sensu stricto, B. afzelii, B. valaisiana, B. lusitaniae, and six mixed infections were also obtained. Borrelia species were heterogeneously distributed in the different areas.