The dynamics of acetate accumulation was studied in Escherichia coli K-12 batch cultures with a substrate addition. At pH 7.0, the growth stopped at 150 mmoles of acetate per litre of the medium under the aerobic conditions or at 35 mmoles of acetate per litre of the medium under the anaerobic conditions. Experiments with extraneous addition of acetate suggest that acetic acid plays a key role in inhibiting the growth of E. coli by acid metabolites. The authors propose a hypothetical mechanism to account for the inhibiting action of acetate.