Five healthy children under 6 years of age, five healthy adults, and 10 adult periodontitis patients were examined for the prevalence and distribution of black-pigmented Bacteroides in the oral cavity. A total of 13 samples was obtained from each individual, including four supragingival and four subgingival dental plaques, dental occlusal surface, buccal mucosa, dorsal tongue, tonsil, and whole saliva. Black-pigmented Bacteroides were recovered from nine adult periodontitis patients. Healthy adults harbored the organisms in low incidence and proportions, whereas the children exhibited no cultivable black-pigmented Bacteroides. The organisms were isolated in highest proportions from dental plaque, especially subgingival plaque, and from the tonsil area, indicating that these sites constitute the organisms' primary ecological niche in the oral cavity. The predominant isolate was Bacteroides melaninogenicus subsp. intermedius followed by Bacteroides gingivalis and B. melaninogenicus subsp. melaninogenicus. B. melaninogenicus subsp. levii constituted low proportions of supragingival microflora of one adult periodontitis patient. A positive correlation was demonstrated between the proportion of black-pigmented Bacteroides (mainly B. melaninogenicus subsp. intermedius) and both the severity of gingival inflammation and the periodontal pocket depth, suggesting that these organisms may contribute to the pathogenesis of certain forms of periodontal disease.