We characterized 52 anaerobic, gram-negative, nonpigmented, saccharolytic rods that were isolated from healthy and diseased subgingival sites of 16 family-owned dogs with spontaneous, clinically diagnosed periodontitis. Phenotypic features were determined with use of standard biochemical methods, by enzymatic profiling with the API ZYM system, and by cellular fatty acid profiling. Genotypic characterization was performed by DNA-DNA hybridization. Four phenotypic groups, defined as the Bacteroides fragilis group, the Prevotella buccae-like rods, the Prevotella heparinolytica/Prevotella zoogleoformans-like rods, and the slimy bile-tolerant rods, designated the Bacteroides pyogenes/Bacteroides tectum group, were detected. P. buccae and the B. pyogenes/B. tectum group organisms were isolated significantly more often (P values, < .05 and < .005) from the diseased than the healthy subgingival sites. The phenotypically similar group of bile-tolerant organisms, including B. pyogenes and B. tectum, most likely constitutes a major component of the anaerobic, gram-negative, saccharolytic microflora in periodontal lesions in dogs, a flora different from that in humans.