A total of 74 strains of oral treponemes, which were isolated from subgingival plaque samples from patients with periodontitis, were taxonomically studied on the basis of biochemical characteristics, DNA-DNA hybridization, and 16S rRNA gene sequences. These organisms fermented carbohydrates and required rumen fluid or short-chain volatile fatty acids for growth. The isolates were divided into seven subgroups based on their biochemical characteristics. The levels of DNA relatedness among the representative strains of each subgroup and Treponema socranskii (including three subspecies) were greater than 78%, while the levels of DNA relatedness among these strains and other Treponema species, including T. denticola and "T. vincentii", were less than 15%. DNA-DNA hybridization indicated that all subgroups belonged to T. socranskii. This result correlated well with the cluster on the phylogenetic trees based on 16S rRNA sequences.