It has been stated that differences in lactobacillus counts may be only a measure of the difference in the number of open carious lesions.' On the other hand, it has also been stated that the percentage distribution of high and low lactobacillus counts reflects the intensity of caries attack and is little influenced by the number of open lesions.2' 3 Evidence has been presented in two preliminary reports based on limited data from the Evanston Fluoridation Study, which strongly indicates a direct relationship between the average number of untreated various surfaces per child and the salivary lactobacillus counts.4' 5 This paper is founded on 2,991 continuous resident children six to eight and twelve to fourteen years of age. One salivary specimen per child was collected from children examined over an 11-year period according to the protocol of the Evanston Fluoridation Study. The study area, Evanston, Illinois, and the control area, Oak Park, Illinois, are alike in most all respects, and both receive water from Lake Michigan. Salivary specimens collected in Evanston in 1946 were collected prior to the addition of fluoride to the Evanston communal water supply. The salivary specimens collected in Oak Park, the control area, were collected in 1947 and again in 1956 prior to the addition of fluoride to the Oak Park water supply. Beginning with 1948 and continuing through 1955, the collections were made in Evanston, Illinois, where the communal water supply contained fluorine as sodium fluoride in the