Acetic acid inhibited the formation of cytochromes and the respiratory activity of the halo-tolerant yeast Zygosaccharomyces rouxii R-l. The growth of the yeast was significantly inhibited by 0.5% acetic acid in a medium containing 18% NaCl. This growth inhibition by acetic acid is thought to be related to the halo-tolerance of the yeast. Proton expulsive activity, which is necessary for the yeast cells to be able to tolerate and grow in a high-saline environment, was markedly lowered in the presence of 0.5% acetic acid, although the plasma membrane ATPase, which drives proton expulsion, seemed not to be inhibited by 0.5% acetic acid. It is assumed that halo-tolerant yeasts are less tolerant to high concentrations of NaCl in the presence of acetic acid because of the reduction of proton expulsive activity, thereby inhibiting growth.