The investigation to be reported was undertaken to determine the time of occurrence, characteristics, and distribution of carious lesions in the molar teeth of Syrian hamsters and to compare the apparent mechanisms of dental caries in this animal -with that described in other species, and in man. EXPERIMENTAL Group A. Eighty-five hamsters (55 females and 30 males) about 35 days of age were selected from 16 litters raised on Purina Rabbit Checkers and fresh green alfalfa. All animals were marked, distributed into cages containing wood shavings, and placed on a high carbohydrate diet of whole wheat flour 20%, corn starch 25%, confectionery sugar 20%, whole powdered milk 30%, and alfalfa 5%. Supplements of fresh carrots or alfalfa were provided once a week. The prepared diet and tap water were available ad libitum. Twenty older animals (10 females and 10 males), taken from breeding stock, were examined for caiies and periodontal disease under ether anesthesia and placed on a similar regimen.With few exceptions, 4 male and 7 female animals were sacrificed every 10 days cver an 81 day period. After fixation in 10% formalin, the jaws and teeth were examined under a low-power dissecting microscope (1.5 X 10) for stains, debris accumulations, plaque-like material, and gingival disturbances. The soft tissues were then stripped from the jaws and the teeth examined for evidence of caries activity. All findings were recorded on special charts. Numerous ground and decalcified sections were prepared, especially of those teeth which showed incipient lesions.Group B. Because of the early appearance of carious lesions, some of the work was repeated 1 year later by placing 21 newly weaned animals on a regimen similar to that described except that no supplement of greens was provided. Six animals were sacrificed before the experiment; 7 animals, at 11 days; and 8 animals, at 20 days. Gross and microscopic examinations of the teeth were