The morphology of human dental plaque has been studied with the electron microscope. In the inner part of the plaque, especially in the region adjacent to the tooth surface, many microorganisms exhibit unusually thick cell walls and contain large amounts of intracellular polysaccharide. The magnitude of both phenomena diminishes towards the saliva-plaque interface. Both cell wall thickening and intracellular polysaccharide storage are considered to be manifestations of a so called ‘unbalanced growth’ situation, suggesting a low or negligible rate of cell multiplication.