Semi-purified diet 2700 caused a relatively infrequent production of carious lesions on the smooth surfaces of the molars of Harvard and mutant albino rats which are susceptible to high incidences of carious lesions in the occlusal sulci. The use of several diets composed mainly of white bread and whole wheat flour resulted in substantial increases in the incidence of smooth surface lesions. In some cases these diets simultaneously caused significant decrease in the incidence of lesions in the occlusal sulci, while in other situations the incidence of both types of lesions was simultaneously high. The rats of the mutant albino strain tend to be more prone to develop the carious lesions on the smooth surfaces than rats of the Harvard strain. In addition, the distribution and characteristics of the smooth surface lesions tended to differ between the 2 strains, although the responses to the various diets were closely parallel.