1. During the two-year period, caries developed at 20% of the target premolar sites. The attack rate for these surfaces was similar in the plaque panel and the other subjects in the study. 2. The microbial composition of plaque samples from caries-free sites and from carious sites before and after radiographic detection of lesions was broadly similar. 3. Numerical domination of particular sites by S mutans before detection of caries can occur, but has only been observed so far in 2 of 15 sites. 4. Pooled date from sites which have developed lesions indicate a rise in the isolation frequency and mean numbers of S. mutans after detection of caries. This trend was particularly obvious in the one subject who developed bilateral lesions by the second examination and in three of four sites where caries was detected at the fourth examination. Similar observations have been made with lactobacilli. 5. In two of 15 instances no isolations of S mutans were made from sites which developed caries. 6. To date, no single species appears to be uniquely associated with the onset of dental caries.