When yeast grows on media in which a final external pH lower than 4 is attained there is a 2-3-fold increase in plasma membrane ATPase activity. This acid-mediated activation produces a 2-fold increase in the ATPase affinity for ATP but does not modify its optimum pH. The acid-mediated activation is dependent on the stage of growth, only late logarithmic or stationary cells being capable of being activated by incubation in an acidic buffer. Only cells able to activate the ATPase maintain a constant internal pH when incubated in acidic buffers. It is concluded that acid-mediated activation of the plasma membrane ATPase is a mechanism of maintaining constant internal pH during growth of yeast on acid media.