The caries–inhibiting effect of the extract from shiitake (Lentinus edodes), the most popular edible mushroom in Japan, was studied both in vitro and in vivo. Shiitake extract showed an inhibitory effect on water–insoluble glucan formation from sucrose by crude glucosyltransferases of Streptococcus mutans JC–2 and Streptococcus sobrinus OMZ–176. The firmly adherent plaque in the artificial plaque formation test was strongly inhibited by shiitake extract. The reduction of firmly adherent plaque caused an increase in the incidence of non– and loosely adherent plaque and a decrease in total plaque formation. A significantly lower caries score was observed in specific pathogen–free rats infected with S. mutans JC–2 and fed with a cariogenic diet containing 0.25% shiitake extract as compared with controls fed the cariogenic diet without shiitake extract.