Proton-pumping activity of a Zygosaccharomyces rouxii , AN 39, grown in YPD medium decreased extensively in 15% NaCl solution, while that of a Saccharomyces cerevisiae, NA 87-11 A, declined in 8% NaCl. However, ATPase of the plasma membrane from NA 87-11 A was somewhat more tolerant toward NaCl than that from AN 39. AN 39 cells grown in YPD medium containing 5% NaCl showed high proton-pumping activity in 15% NaCl, whereas NA 87-11 A cells grown in the same medium did no increase of proton-pumping activity in 10% NaCl. NA 87-11 A cells incubated in 10% NaCl solution accumulated smaller amount of ATP, that is, less than one-third of AN 39 cells. A NaCl-hypersensitive mutant of Z. rouxii, 152 S, which was defective in a Na+-excretion mechansim and could not grow in YPD medium containing more than 1% of NaCl, showed approximately the same level of ATP and proton-pumping activity in 10% NaC1 solution as the wild type, AN 39. On the other hand, another salt-sensitive mutant of Z. rouxii showing altered sterol composition, TN 3, accumulated high level of ATP whereas exhibited low proton-pumping activity in 10% NaCl solution, although NaCl tolerance of ATPase of plasma membrane from this mutant was the same as that from AN 39. These results suggest that the low proton-pumping activity of plasma membrane ATPase and low level of intracellular ATP of NA 87-11 A in NaCl solutions of high concentrations are concerned with low salt-tolerance of S. cerevisiae. It is also suggested that the proton-pumping activity of plasma membrane ATPase in intact cells incubated in NaCl solution is not correlated with the salt-tolerance of ATPase in isolated plasma membrane, and that plasma membrane composition, such as lipid composition, is important in the salt-tolerance of yeast, since it may affect the proton-pumping activity.