The use of glucose as the carbon source during the growth of Pichia stipitis NRRL Y7124 increased the specific activity of the plasma membrane ATPase three-fold in comparison to the activity measured with xylose as the carbon source. Compared to the nonactivated enzyme, the glucose-activated ATPase had the same pH and temperature optima, a comparable affinity for ATP, a greater sensitivity to orthovanadate, and a lower sensitivity to ethanol. Furthermore, the ethanol concentration resulting in growth inhibition depended on the nature of the sugar consumed : 30 or 34 g l(-1), respectively, when culture was grown on xylose or glucose. These results showed a correlation between plasma membrane ATPase activity in cells grown either on D-glucose or on D-xylose, and their ethanol tolerance, evaluated by the inhibitory effect of ethanol on growth. Finally, these data, added to the in vitro effect of ethanol on the plasma membrane ATPase, suggested that a decrease of ATPase activity to <60% of the control (measured without alcohol in the test) would prevent the growth of P. stipitis.