Summary 1. No evidence of dental caries was found in 365 animals fed the normal diets for periods of 2 to 18 months. 2. Seventy out of seventy-one animals fed a diet low in phosphorus and vitamin D for periods of 2 to 7 months showed dental caries. This diet, composed largely of corn and wheat gluten, is also low in other minerals and vitamins, and the proteins are not of a good quality. 3. The addition of vitamins A, B1, B2, C and E to this diet apparently did not have any effect on the development of dental caries. 4. The addition of phosphorus to this diet prevented the development of dental caries in 4 out of 7 rats when fed for periods of 2 3/4 to 6 months. 5. The addition of vitamin D to this diet in the form of viosterol prevented the development of dental caries in 6 out of 19 rats when fed for periods of 2 1/2 to 5 months. 6. Ten animals were fed a diet composed of cornstarch, casein, vegetable fat (crisco) and yeast, with added minerals and vitamins for periods of 5 1/2 to 13 months. This diet, although low in phosphorus and vitamin D, is adequate in other respects. Dental caries developed in 5 of these animals. 7. Nine animals fed this diet with the addition of vitamin D in the form of viosterol for periods of 6 to 13 months did not show evidences of dental caries in any instance. 8. The amount of calcium in the diet apparently had no influence on the development of dental caries. 9. A preliminary examination of a number of children under observation indicates the importance of an adequate supply of vitamin D for the prevention of dental caries.