Denture plaque has been studied by transmission electron microscopy in glycolmethacrylate-embedded specimens from six patients presenting with typical denture stomatitis. Different types of thin and thick pellicles were found applied to the denture surface. These were in contact with a denture plaque made up predominantly of loosely packed Gram- positive and Gram- negative bacteria. Rounded, rod-shaped and filamentous micro-organisms were all evident. Group of Candida albicans with normal ultrastructure were noted in some limited zones. When surrounded by bacteria, especially by bacteria in corn-cob configurations the Candida showed cytological signs of degeneration. In one case out of six a bacterial penetration of the acrylic denture base was observed, through narrow and straight channels and subsurface cavities filled with micro-organisms. Bacterial penetration was also seen in the most superficial cell layers of the palatal mucous membrane.