Recent taxonomic and anatomical studies of dental plaque associated with periodontal health and disease have demonstrated that differences in the microbial populations in plaque may be responsible for the initiation and progression of disease. The consistent isolation of large numbers of anaerobic and capnophilic bacteria from the depths of periodontal lesions has suggested an important role for these organisms. Bacteria that have been isolated include Capnocytophaga (Bacteroides ochraceus), other species of Bacteroides, Fusobacterium, Selenomonas, spirochetes, Campylobacter, Veillonella, Actinomyces, Propionibacterium, Peptococcus, and other genera. The periodontopathic potential of oral strains of Bacteroides melaninogenicus has been explored in a number of investigations because these organisms are consistently isolated from periodontal lesions. Studies of B. melaninogenicus have included purification of a capsular substance, characterization of the lipopolysaccharide and a variety of toxic substances and lytic enzymes, and ecologic aspects of its colonization. Understanding of the nature and pathogenic mechanisms of the oral microbiota may lead to control of this pandemic infection.