Four hundred and sixty-two children, aged 10-14 years, from eight areas were examined for Streptococcus mutans in saliva and for dental caries. Strep. mutans strains from some samples were further characterized by serological and biochemical methods. Strep. mutans was identified in 98 per cent of the children and 40 per cent showed high counts. Serotypes c and d dominated among the strains. The distribution of Strep. mutans among the children varied between the eight areas but was not correlated with the difference in caries prevalence which ranged from 38 to 88 per cent. In individual children, however, there was an association between high counts of Strep. mutans and the presence of dental caries. Thus, Strep. mutans can be widely distributed and occurs in high numbers in a population with a low prevalence of caries.