Among the three main species of Borrelia burgdorferi sensu lato associated with Lyme borreliosis, B. burgdorferi sensu stricto (B. burgdorferi) is the sole species present both in North America and in Europe, where Borrelia garinii and Borrelia afzelii also occur. The greater genetic diversity together with the greater clinical polymorphism observed in the Old World suggests that this is the birthplace of the complex B. burgdorferi sensu lato. However, the genetic proximity of some North American and European B. burgdorferi strains in quite mystifying. A previous study of the whole genome (M. Foretz, D. Postic, and G. Baranton, Int. J. Syst. Bacteriol. 47:11-18, 1997) compared the diversity of North American and European B. burgdorferi strains. To further investigate the geographical origin and the migration of B. burgdorferi, we have focused on the study of the single variable and highly adaptive gene ospC. Both approaches demonstrated the greater diversity of North American strains and the close relatedness between European strains and between some isolates from the two areas. We discuss the significance of these features and suggest that they might be evidence of the anteriority of North American B. burgdorferi strains.