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Borrelia burgdorferi sensu lato is present all over the world. However, most species belonging to this complex have a limited distribution; the only exception is B. burgdorferi sensu stricto, which is encountered both in Europe and in the United States. The aim of this study was to clarify the way that B. burgdorferi sensu stricto migrated. To do this, we compared the genetic polymorphism of 33 B. burgdorferi sensu stricto strains isolated in Europe and 28 strains isolated in the United States by using both pulsed-field gel electrophoresis and arbitrarily primed PCR. Phylogenetic analysis of each of the two sets of data was performed by a genetic distance method and a parsimony method. The results were consistent and revealed that the American strains were more heterogeneous than the European strains. The hypothesis that B. burgdorferi sensu stricto was introduced to Europe from America rather than vice versa has to be considered.