The aim of this study was to test the hypothesis that suspected periodontal pathogens form a minor component of the supragingival plaque of individuals without periodontal diseases. Twenty-one dental hygienist trainees with a mean age of 23.5 years were twice sampled for interdental plaque between 1st and 2nd molars in all quadrants. The samples were assessed for Actinobacillus actinomycetemcomitans, Bacteroides forsythus, Campylobacter rectus, Porphyromonas gingivalis, and the Prevotella intermedia/Prevotella nigrescens group of organisms. Bacteria of this group were predominantly P. nigrescens and showed both the highest prevalence (100%) and the highest colonization density of the investigated species. Seven of 21 samples harbored A. actinomycetemcomitans. Serotypes a, b, and c were found in three samples each, while serotype e was present in one sample. Three subjects had two different A. actinomycetemcomitans serotypes. Bacteroides forsythus and C. rectus were detected in 10 (48%) and nine (43%) subjects, respectively. The detected cell numbers accounted for approximately 0.01% to 1% of the sampled flora. In contrast, P. gingivalis was found only in a single sample, which in addition harbored B. forsythus, C. rectus, A. actinomycetemcomitans (serotypes b and e), and P. intermedia. These results suggest that the investigated periodontal bacteria are not "exogenous pathogens", but amphibiotic, opportunistic microorganisms which may have a natural habitat in the supragingival plaque of the interproximal area of molars.