Treponema strain PFB4G is a novel oral spirochete and one of the most frequently detected organisms in subgingival plaque samples from rapidly progressive periodontitis and adult periodontitis patients. In this study, a genomic library of Treponema strain PFB4G was constructed in expression vector. One positive clone that carried a 2.6-kb fragment was identified by screening with chicken Ig Y (immunoglobulin yolk) antibody raised against whole bacterial sonicates. Nucleotide sequencing of the subclones revealed an open reading frame (ORF) lacking the 5'-end. This region was obtained by PCR amplification using a degenerative and a specific primer. A complete open reading frame of 1,770 bp was identified and the deduced polypeptide consisted of 590 amino acids with a molecular mass of 65 kDa. The polypeptide, designated as OmpTL, had a typical prokaryotic signal sequence (19 amino acids) with a potential cleavage site for signal peptidase I and showed a significant level of homology with the outer membrane proteins of other oral treponemes, especially with that of Treponema maltophilum. The isolation of the gene encoding an outer membrane protein may allow the study of their roles in future, possibly as adhesion, pore forming or induction of proinflammatory cytokine synthesis.