Two groups of Thai schoolchildren aged 11-13 yr were examined for dental caries, dental fluorosis and Streptococcus mutans. One hundred children lived in an urban district, Bangkok, and 71 children in a rural district, Petchaboon. Saliva samples were analyzed for S. mutans by the spatula method. The fluoride content of the drinking waters was also determined. The prevalence of caries, diagnosed according to WHO, was 89% in Bangkok and 18% in the rural district. Mean DMFT was 3.46 and 0.38 respectively. S. mutans was found in 98% of the urban children and in 82% of the rural. The differences in distribution between S. mutans and DMFT classes were statistically significant for the total sample. Mild fluorosis was present in some of the rural children.