The morphologic structure of anorganic dental calculus was studied by means of the scanning electron microscope. From surface observations, calculus is apparently composed of two components with distinguishable patters of calcification. One component is formed by the precipitation of minute calcific crystals on microorganisms and intermicrobial substances (plaque matrix). Such calcified masses, often spherical in shape, have a sponge-like appearance with empty spaces representing the former sites of entombed and degenerated organisms. Thus, intracellular calcification is not evident at this stage of calculus development. The other component, although having at least one common calcification front with the former, does not appear to be directly associated with microbial calcification. It exhibits a configuration of generally larger crystal growths of varying shapes and sizes. These two calcification patterns are comparable, both in distribution and size, to what has been observed by means of the transmission electron microscope, and what Schroeder has designated as "types A & B centers of mineralization," respectively. The calcific precipitation in type A centers have been identified by X-ray diffraction as hydroxyapatite. It is, therefore, speculated that the crystal patters in type B centers might represent other known forms of calcium phosphates present in calculus, such as octacalcium phosphate, whitlockite and brushite.