The species of sulfate-reducing bacteria that prevail in sites affected by periodontal disease may be different from those commonly occurring in the digestive tracts of healthy individuals. Ten strains of mesophilic sulfate-reducing bacteria (SRB) were isolated from subgingival plaque in periodontal lesions of ten patients with periodontitis. Characterization on the basis of morphological, physiological and phylogenetic properties demonstrated two distinct types of oral SRB. One strain was a curved rod with high motility. For dissimilatory sulfate reduction, lactate or pyruvate was oxidized incompletely to equimolar amounts of acetate. Desulfoviridin and cytochrome c3 were present in this mesophilic vibrio and the cellular lipid profile was similar to that from members of the genus Desulfovibrio. The 16S rDNA sequence was similar to that of the proposed 'Desulfovibrio fairfieldensis'. Cells of the nine other strains were straight, rod-shaped, exhibited a low growth rate and oxidized substrates incompletely to acetate. These SRB, like members of the genus Desulfomicrobium, lacked desulfoviridin. Analysis of the 16S rDNA sequences of seven of the nine isolates showed a high degree of similarity among these oral strains, forming a distinct lineage within the genus Desulfomicrobium. The cellular lipid profile of a representative oral strain, NY678T, was in accordance with that of other Desulfomicrobium species, but also showed dissimilar features. The phenotypic and phylogenetic analyses indicate that these rod-shaped SRB from the oral cavity could be regarded as a new species, for which the designation Desulfomicrobium orale sp. nov. is proposed.