Acid production, in particular at low pH, is thought to be an important ecological determinant in dental caries. The aim of the present study was to determine the acid producing capability at different pH levels of 47 streptococcal strains, representing 9 species, isolated from human dental plaque. The bacteria were grown until mid log-phase under anaerobic conditions and acid production was measured in a pH-stat system at pH 7.0, 6.0, 5.5 and 5.0. At all pH values, the mean velocity of acid production (V(ap)) by Streptococcus mutans and S. sobrinus was significantly higher (p<0.01; ANOVA) than that of the other oral streptococci, including S. mitis, S. oralis, S. gordonii, S. sanguis, S. intermedius, S. anginosus, S. constellatus, and S. vestibularis. However, the V(ap) of some strains of S. mitis biovar 1 and S. oralis, particularly at pH values of 7.0 and 6.0, exceeded that of some strains of S. mutans. The V(ap) decreased with pH for all strains, but some strains of S. mitis biovar 1 and strains of the mutans streptococci maintained a relatively high rate of acid production. The results suggest that some strains of S. mitis biovar 1 and S. oralis may play an important role in caries development by modifying the environment in dental plaque to become favourable for the succession of aciduric species. The study furthermore emphasises the need for detailed species and biovar identification of oral streptococci and for recognition of the significant physiological differences that occur within single species.