The partial 16S rRNA gene sequences of representative strains of two groups of anaerobic, gram-negative, pigmented, asaccharolytic, rod-shaped bacteria isolated from subgingival plaque of dogs with naturally occurring periodontal disease were determined. A comparative analysis of the rRNA sequence data revealed that the two groups of organisms represent previously unknown lines of descent within the genus Porphyromonas. On the basis of our phylogenetic findings and the phenotypic distinctiveness of the organisms, two new species, Porphyromonas cangingivalis and Porphyromonas cansulci, are proposed.