The morphology of supragingival and subgingival calculus on extracted teeth was studied with the scanning electron microscope. Oral, crevicular, and fracture surfaces were examined. Both subgingival and supragingival calculus had a heterogenous core covered by a soft, loose layer of microorganisms. On supragingival calculus this layer was dominated by filamentous microorganisms while subgingival calculus was covered by a mixture of cocci, rods and filaments. The supragingival covering of filaments was oriented with the filaments approximately perpendicular to and in direct contact with the underlying dense calculus. This arrangement was rarely seen subgingivally where there was no distinct pattern of orientation. Some of the specimens of sub- and supragingival calculus were treated with sodium-hypochlorite. These lost the soft covering, and channels the same size as the filamentous organisms were found penetrating into the calculus. They were oriented prependicular to the surface in supragingival calculus but had no specific direction in subgingival calculus. The appearance of the channels supports the concept that calcification starts between the microorganisms in both subgingival and supragingival calculus.