Nine oral bacteria, associated with both healthy and diseased sites in the mouth, were grown at = 0·05 h (mean generation time 13·9 h) in a glucose-limited chemostat. After an initial period of steady-state growth at pH 7·0, pH control was discontinued. The pH then decreased until it stabilized at pH 4·1 after 9 d (16 generations), while the rose from -165 mV to +160 mV. The lowering in pH resulted in the composition and metabolism of the flora being altered and in increased bacterial aggregation. At pH 7·0, ‘’, and were most numerous while at pH 4·1 the counts of all bacteria fell except for , which became predominant. The proportions of within the community also increased while was recovered only occasionally and was not detected below pH 4·6. The survival at pH 4·1 of several other species would not have been predicted from earlier pure culture studies. Relative to pH 7·0, the community growing at pH 4·1 produced more lactic acid, washed cells had a greater glycolytic activity over a wider pH range but amino acid metabolism decreased. In general, when pH control was restored, so were the original patterns of metabolism and bacterial counts, except for , which was still not detected. The inverse relationship between and , and the increase in proportions of and during growth in a low H environment parallel observations made and suggest that the chemostat can be used as a model for microbial behaviour in dental plaque.