THE ULTRASTRUCTURE of nondecalcified supragingival and subgingival calculus was studied in mature deposits. To facilitate sectioning of the embedded material, a thin reinforcing film of plastic was painted on the block. A new film was applied for each section. Light microscopy showed that supragingival calculus was heterogeneous with islets of calcified material within the covering plaque and with noncalcified areas within the calculus. Under transmission electron microscopy supragingival calculus was heterogeneous, dominated by microorganisms, small needle-shaped crystals and large ribbon-like crystals. In the covering soft plaque small crystals were often scattered in the intermicrobial matrix. In the supragingival calculus itself noncalcified microorganisms were surrounded with densely packed small crystals. There were also rosettes and bundles of large crystals. Subgingival calculus was homogeneous in light microscopy. The covering plaque contained no calcified material and only calcified material was seen within the calculus itself. Transmission electron microscopy of subgingival calculus revealed crystals of small size only. Subgingivally very few noncalcified microorganisms were seen within the calculus. The bacterial cell wall seemed to be the structure that was last calcified, both supragingivally and subgingivally.