An enriched nutrient agar medium containing blue dextran has been utilized for the detection of dextranase-producing microorganisms in human dental plaque. When compared with the total viable anaerobic plaque flora, the proportion of these microbes in supragingival plaque from different individuals varied over a wide range. Preliminary characterization of some of the dextranase-producing microorganisms revealed a heterogeneous mixture of cell types with varying morphological and biochemical characteristics. Several bacterial isolates were tentatively identified as being members of the genus Actinomyces. An additional isolate appeared to belong to the genus Bacteroides. The dextran-degrading enzymes produced by these bacteria are extracellular, and a cell-free preparation from one of the isolates has been shown to cause extensive endohydrolytic cleavage of high-molecular-weight dextrans.