AbstractIt has been shown that addition of phosphate to phosphate deficient yeast gives rise to an immediate increase in the rate of Na+uptake and an immediate decrease in the rate of Rb+uptake. In addition, phosphate uptake is enhanced specifically by Na ions presumably by a process with a very high affinity for phosphate with aKmof about 2 × 10−6Mat pH 7.2, whereas theKmfor phosphate uptake of the Na+independent process amounts to 1.3 × 10