The formation of water-insoluble glucan by extracellular glucosyltransferase from Streptococcus mutans 6715 found to be greatly stimulated by various mono- or divalent cations. An enzyme preparation, obtained by ethanol fractionation, was able to catalyze the formation of water-insoluble glucan from sucrose in the presence of monovalent cations above 100mM or divalent cations above 20 mM at neutral pH. As the concentration of monovalent and divalent cations was reduced to below 10 mM and 1 mM, respectively, the formation of insoluble glucan decreased to a negligible amount. High concentrations of these cations were found to stimulate the formation of insoluble glucan in the following ways: (i) it increased the activity of total glucosyltransferase up to 1.6- and 2.7-fold in the absence and presence of a primer dextran, respectively, and (ii) it changed the formation of soluble glucan to insoluble. It was postulated that one of the essential factors for the formation of insoluble glucan would be to keep more than two water-soluble glucan chains close to enzyme aggregates and that such interaction could be enhanced by the presence of high cation concentrations.