It has been demonstrated by Arnold (1), by Dale, Lazansky and Keyes (2), and by more recent observations (3) in this laboratory that the Syrian hamster, Cricetus auratus, when maintained on various kinds of soft diets will develop lesions in the molar teeth which closely resemble dental caries in man. It seems apparent, as has been demonstrated many times with other species, that bacteria must have an important role in relation to the effects of caries-producing foods on the teeth of the hamster. Since there were no reports in the literature on the subject, this study was undertaken as'a preliminary investigation of the oral bac-