ABSTRACT Saccharomyces cerevisiae plasma membrane H + -ATPase activity was stimulated during octanoic acid-induced latency, reaching maximal values at the early stages of exponential growth. The time-dependent pattern of ATPase activation correlated with the decrease of cytosolic pH (pH i ). The cell population used as inoculum exhibited a significant heterogeneity of pH i , and the fall of pH i correlated with the loss of cell viability as determined by plate counts. When exponential growth started, only a fraction of the initial population was still viable, consistent with the role of the physiology and number of viable cells in the inoculum in the duration of latency under acid stress.