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A method was presented to fabricate epoxy resin crowns to be worn by human subjects requiring full crown restorations. These crowns were utilized in six young adults to study the internal structure of plaque after plaque formation periods of 1 and 3 days, 1 and 3 weeks and 2 months. This study confirmed previous findings that early plaque contains primarily coccal forms, with a shift to predominantly filamentous forms by 3 weeks. Early plaque growth seems to occur by the formation of columnar microcolonies which coalesce and grow by cell division within the colony in a direction perpendicular to the crown surface. Filamentous microorganisms appear in large numbers by 1 week. They appear to colonize the surface of the predominantly coccal plaque, eventually growing into it and replacing the coccal forms. The subgingival, mature plaque contains many motile forms including bacteria with unusual cell wall ultrastructures. Certain bacteria combine into distinctive bacterial aggregations resembling "corn cobs" and "test tube brushes," the latter occurring exclusively in subgingival plaque. Spirochetes appear to grow preferentially on the external surface of subgingival plaque in close contact to the gingival tissue of the deepened sulcus. Their high concentration in the external layer of subgingival plaque suggests that because of their strategic location they may play an important role in the etiology of periodontal disease. Studies of well preserved plaque, possibly combined with the use of serological markers, can serve a useful role in identifying certain microorganisms in dental plaque. Because of their numbers and/or location in relation to periodontal tissues, some of these bacteria may warrant further studies as potential etiologic agents of certain forms of periodontal disease.