Teeth slated for extraction were evaluated with respect to their periodontal status and classified accordingly into five categories; namely normal, gingivitis, periodontitis, periodontosis and postperiodontosis. After processing, one approximal surface of each tooth was sampled at various levels in an apico-occlusal direction for light and electron microscopic study of the associated bacterial flora. In normal samples, the flora consisted of a relatively thin, adherent bacterial layer confined to the enamel surface. The cells were predominantly coccoid in shape with cell wall features compatible with those of Gram-positive organisms. Isolated filamentous or branching forms and some Gram-negative bacteria were noted on the surface of the more apical portion of the bacterial layer. No flagellated cells or spirochetes were present. Gingivitis samples yielded a relatively more voluminous and complex supragingival flora with relatively more filamentous bacteria and more cells with a Gram-negative cell wall ultrastructure. These samples also contained corncob formations on the surface of supragingival deposits, and flagellated cells with spirochetes within the predominantly Gram-negative flora of the sulcus bottom. Supragingival bacterial deposits of periodontitis samples were similar to those observed in gingivitis. The subgingival flora consisted of relatively fewer cells adherent to the root surface with a concomitant increase in the population of Gram-negative and flagellated cells, as well as spirochetes. The tissue side of the subgingival flora generally exhibited a distinctive concentration of "test-tube brush" formations, spirochetes of predominantly medium size, and assorted cell types peculiar to this region. A transitional flora generally separated the supra- from the subgingival microbial population. Periodontosis samples had a relatively sparse, predominantly Gram-negative flora. A unique electron-dense, lobulated cuticular deposit covered the majority of the samples studied. Postperiodontosis samples were much more similar in their microbial flora to the periodontitis group. The results suggest that (1) a certain microbial flora may be compatible with a state of periodontal health; (2) a different flora is associated with varying degrees of periodontal disease; (3) the structure and composition of the supragingival flora differs markedly from that of the subgingival flora; (4) with the exception of periodontis, the alterations of the microbial flora as periodontal disease increases inseverity parallel the changes described previously in the microbial population collected on artificial crowns during experimentally induced gingivitis. The use of the expressions "microbial flora" or "microbial population" is considered preferable to the terms "plaque", "materia alba", or "debris" in reference to the microbiota of the gingival sulcus region.