Abstract: Nucleic acid fragments are disclosed which encode a polypeptide antigen reactive with antisera from rabbits immunised with a 66 kDa protein from Borrelia garinii IP90. The presence of nucleic acid fragments encoding such a polypeptide antigen as well as the presence of the polypeptide antigen have been demonstrated in three strains of B. burgdorferi sensu lato, but are substantialle absent from at least 95% of randomly selected B. hermsii, B. crocidurae, B. anserina, and B. hispanica. The encoded polypeptide is surface exposed on the bacterial surface, it is highly conserved, and is thus potentially useful as a vaccine agent and as a diagnostic agent in the diagnosis of infections with B. burgdorferi as are the characteristic nucleic acid fragments of the invention. Also disclosed are methods of producing the polypeptide antigen according to the invention as are antibodies directed against the antigen.