We have examined the organization of ospA and ospB, genes encoding the two major outer membrane proteins of the Lyme disease spirochete, Borrelia burgdorferi. These genes were each subcloned separately from a recombinant plasmid containing both genes. DNA sequence analysis of protions of these genes and determination of the location and direction of transcription of each osp gene indicated that these genes are co-transcribed. Northern blot analysis of mRNA from B. burgdorferi with DNA probes individually specific for ospA or ospB identified a single, large transcript that hybridized with each probe.