The prevalence of Lyme disease Borrelia-infected ticks on migrating birds was studied in Scandinavia. A total of 22,998 birds were caught at eight different bird observatories and examined for ticks. Five different species of ticks were found infesting the birds. The dominant species, Ixodesricinus, constituted 98.3% of the ticks collected. The presence of spirochetes was determined by an immunofluorescence assay of tick larvae and DNA amplification by PCR on all ticks. To determine which Borrelia burgdorferi sensu lato species were present, a species classification was performed by DNA amplification with species-specific 16S rDNA primers and by DNA sequencing (rDNA is DNA coding for rRNA). Flagellin gene sequences of all species of B. burgdorferi sensu lato previously recorded in Europe were observed. Borrelia garinii was the most prevalent Lyme disease Borrelia species in ticks collected from birds arriving from the South or Southeast in the spring, whereas the distribution was more heterogeneous in ticks from birds migrating from the Southwest. These data support the notion that birds are partly responsible for the heterogeneous distribution of Lyme disease Borrelia spirochetes in Europe.