This study was carried out to investigate the predominant anaerobic bacteria of periodontal pockets in patients with advanced periodontitis, who had no previous treatment other than supragingival scaling, no history of recent or chronic systemic illness, nor any intake of antibiotics within 6 weeks prior to bacteriological sampling. Care was taken not to ignore tiny-colony-forming anaerobes, by means of a stereoscope and an anaerobic glove box system. Out of 422 (100%) isolates, 380 (90%) were obligate anaerobes, suggesting that the environment in periodontal pockets was anaerobic and favors the growth of obligate anaerobes. Among the 380 obligate anaerobes isolated, strains belonging to Eubacterium (54%) were predominant, and many of them occurred in tiny colonies. The other obligate anaerobes isolated were assigned to Wolinella (9%), unidentified motile rods which resemble Wolinella (7%), Peptostreptococcus (6%), Fusobacterium (5%), Bacteroides (2%; including those reclassified to Prevotella and Porphyromonas) and Selenomonas (0.5%). Among the isolates, 67% were Gram-positive bacteria, including 59% of rods (mostly asaccharolytic Eubacterium), suggesting that these bacteria, particularly strains of the Eubacterium species, may play an important role in etiology of adult periodontitis.