Agarose gel electrophoresis of whole genomic DNA of the oral spirochaete Treponema denticola has revealed a plasmid-like fraction. Purification and restriction enzyme analysis has confirmed the presence of a 2.6-kb circular plasmid, which has been mapped for restriction sites and cloned into the Escherichia coli plasmid pUC18. Southern blot analysis of genomic T. denticola DNA, using the plasmid as a probe, has shown that the plasmid is present only as an extra-chromosomal element. No plasmid-coded recombinant gene product from a PstI insert in pUC18 has been detected in host cells of E. coli by SDS-PAGE or immunoblotting with polyclonal immune rabbit serum to T. denticola. The discovery of this plasmid may provide a useful tool in the application of new molecular approaches in spirochaetal biology.