1. Total ATPase levels were determined in homogenate fractions of baker's yeast, Saccharomyces cerevisiae K and Rhodotorula glutinis. The maximum ATPase activities in 8000 X g supernatant of the three yeast strains were 6.0, 1.9, and 2.2 mmol Pih-1 (gDS)-1, respectively; the activities in the sediment were somewhat higher. Exponential cells of S. cerevisiae K and R. glutinis exhibited higher ATPase levels than did the stationary cells. 2. The total ATPase activity in both yeast species showed a maximum at ph 6.8 a minimum at pH 7.2, and another broader masimum around pH 8.0. 3. No significant NaK-ATPase activity was detected in baker's yeast, in either the exponential or the stationary cells of R. glutinis, and in exponential S. cerevisiae K cells in the pH range of 6.0-9.3. 4. Stationary cells of S. cerevisiae K exhibited, at pH 7.0-8.5, A Na,K-ATPase activity attaining 9% of total ATPase level. 5.3 X 10(-3) M phenylmethyl sulphonyl fluoride had no effect on the total ATPase level in S. cerevisiae and inhibited the activity in R. glutinis by 25%; it did not bring forth any Na,K-ATPase activity apart from that found in its absence. 6. 1.5 M urea lowered the ATPase activity in R. glutinis by 68% but had no effect on S. cerevisiae cells. 10(-5) M dicyclohexylcarbodiimide suppressed the ATPase activity in S. cerevisiae and R. glutinis by 74 and 79%, respectively. Neither agent revealed and additional Na,K-ATPase activity. 7. The comparison of Na,K-ATPase activities with data on K+ fluxes across the yeast plasma membrane suggested that even with the lower flux values the Na,K-ATPase, even if present, would account for a mere 40% of transported ions. The results imply that the active ion transport in yeasts is energized by mechanisms other than the Na,K-ATPase.