The purpose of this study was to determine the distribution of black-pigmenting Gram-negative bacteria in the dentition of periodontitis patients and to examine differences in the microbial composition of samples taken from a series of adjacent sites. Separate subgingival samples were taken from the mesial, buccal, distal and oral aspects of every tooth in 10 subjects. Thus, a total of 927 sites, 84 to 102 per patient, were scored clinically and sampled microbiologically. P. intermedia and P. melaninogenica were found in all subjects. P. gingivalis was found in 7. The organisms tended to be distributed unevenly, giving the impression of clusters of positive samples in certain areas of the dentition. 77% of all samples positive for P. gingivalis were also P. intermedia-positive. Premolars had lower frequencies and mean proportions of P. gingivalis than incisors and molars. In the premolar and molar region, frequencies and mean proportions of P. gingivalis increased with more posterior location. While frequencies indicated a similar topographic distribution of P. intermedia the mean proportions of this organism were more consistent at different locations. Based on a logistic regression model, it was estimated that the probability of detecting P. gingivalis was 34 times higher in any site which had at least 1 P. gingivalis-positive neighboring site. For P. intermedia any site with at least 1 P. intermedia-positive neighboring site had a chance 2.4 times higher of harboring the organism as well. The highest chance of detecting P. gingivalis and P. intermedia existed in deep, oral pockets of molars, which bled upon sampling.