The quantitative analysis of the incidence of black-pigmented Bacteroides (B.P.B.) spp. in 80 human dental root canal infections (56 with acute symptoms and 24 clinically asymptomatic) in 79 adults were studied. Altogether 101 strains including 7 species of B. P. B. were identified. It was found that the infection rate of B. P. B. in symptomatic group (192.86%) was higher than that in asymptomatic group (41.67%). The means of quantity of cultivable B. P. B. (CFU/ml) and percentage of B.P.B. (CFU/ml) in total CFU/ml of bacteria were not significant. But the percentage of asacchrolytic B.P.B. species in B.P.B. positive samples in symptomatic group (73.08%) was higher than that in B.P.B. positive samples in asymptomatic group (40%). These results suggest that there is a close correlation between symptoms and the asacchrolytic species of B. endodontalis and B. gingivalis.