Abstract To suspensions of nongrowing commercial baker's yeast were added various organic acids under anaerobic conditions and low pH. Acetic, propionic, and isobutyric acids elicited carbohydrate breakdown, fermentation, and potassium ion uptake. Formic, lactic, glycolic, succinic, glutaric, citric, and hydrochloric acids did not produce these effects nor did ethanol and acetaldehyde. Various monovalent cations had different inhibitory effects on the production of anaerobic CO 2 by acetic acid. Potassium was most inhibitory, and Na + was least inhibitory. Similarly, K + was most readily transported into the cells, whereas Na + was least readily taken up. The effects of the acids and the implications in K + transport are discussed.