Two characteristic strains (935T, 934U) of B. burgdorferi isolated from Ixodes persulcatus and a wild rodent (Apodemus agrarius) in Korea were selected and analyzed by an immunoblot method using the monoclonal antibodies directed to different epitopes of outer surface protein A (OspA). The reactive pattern of strain 934U with these monoclonal antibodies was identical to that of strains belonging to B. afzelii and that of strain 935T was different from other isolates. Monoclonal antibody (5TEE3) which is specific to strain 935T did not react with any other Western and Japanese isolates. So, it was suggested that there exist at least two groups of B. burgdorferi in Korea. One could be classified as B. afzelii and the other is a divergent group from three known species of B. burgdorferi sensu stricto, B. garinii and B. afzelii.