Information regarding the biology and virulence attributes of Treponema pallidum (Tp) is limited due to the lack of genetic exchange mechanisms and the inability to continuously cultivate this spirochete. We have utilized TnphoA mutagenesis of a Tp genomic DNA library in Escherichia coli (Ec) to identify genes encoding exported proteins, a subset of which are likely to be important in treponemal pathogenesis. We report here the identification and nucleotide (nt) sequence of a 5-kb treponemal DNA insert that contains seven open reading frames (ORFs). The proteins encoded by six of these ORFs have homology with members of a newly described protein family involved in the biogenesis/assembly of flagella and the control of flagellar rotation in Ec, Salmonella typhimurium (St) and Bacillus subtilis (Bs). Certain members of this family are also involved in the export of virulence factors in Yersinia (Yr) spp., St and Shigella flexneri (Sf). We have named these six ORFs fliM', fliY, fliP, fliQ, fliR and flhB'. The operon containing these ORFs has been designated as the fla operon. We hypothesize that the protein products of these genes are involved in the biogenesis/assembly of flagella and the control of flagellar rotation in Tp.