Structural analysis by electronmicroscopy and pulsed field agarose gel electrophoresis of the genome of Borrelia burgdorferi has revealed a 1000 kilobase (kb) chromosome and a unique type of extra chromosomal linear DNA, as well as supercoiled circular DNA. The plasmid DNA ranged in size from 1.5 to 50 kb. Gene mapping of the ospA and ospB genes have placed them on the 49 kb linear plasmid molecule. The osp operon of B. burgdorferi has been extensively characterized at the molecular level.