Abstract: This invention relates to flagella-less strains of Borrelia and to novel methods for use of the microorganisms as vaccines and in diagnostic assays. Although a preferred embodiment of the invention is directed to Borrelia burgdorferi, the present invention encompasses flagella-less strains of other microorganisms belonging to the genus Borrelia. Accordingly, with the aid of the disclosure, flagella-less mutants of other Borrelia species, e.g., B. coriacei, which causes epidemic bovine abortion, B. anserina, which causes avian spirochetosis, and B. recurrentis and other Borrelia species causative of relapsing fever, such as Borrelia hermsii, Borrelia turicatae, Borrelia duttoni, Borrelia persica, and Borrelia hispanica, can be prepared and used in accordance with the present invention and are within the scope of the invention.