Three of 16 isolates of Treponema denticola were found to contain small (2.0-2.7 kb) cryptic plasmids. These were pTD1 from T. denticola ATCC 33520, pTD2 from strain T32A, and pTD3 from strain D3A1. These plasmids were characterized by restriction mapping and cloned into E. coli plasmid pUC19. Extensive homology between these plasmids was revealed by Southern blot, and single-stranded DNA regions were found by neutral Southern blots and S1 nuclease mapping. These plasmids were not found in serovars usually associated with human periodontal disease nor are they universal in all T. denticola strains and serovars.