High cell density culture of a recombinant yeast, Candida utilis KU101, were investigated in a jar fermentor under the fed-batch mode to achieve efficient production of single chain monellin. Cell yields for glucose, ammonia, inorganic phosphate and the trace elements, B³⁺, Cu²⁺, Fe³⁺, I^–, Mn²⁺, Mo⁶⁺ and Zn²⁺ were determined. Fed-batch culture was carried out by a nutrient feeding strategy determined from the cell yields. However, growth of the recombinant C. utilis stopped when the cell density reached about 60 g/l and the monellin productivity was only 3.4 g/l. The inhibition was not caused by accumulation of metabolites, nor by the depletion of glucose, ammonia, inorganic phosphate or trace elements during the cultivation. By analyzing the supernatant of the culture broth in detail, it was shown that cell growth was slightly inhibited by concentrations of sodium or potassium ions above 100 mM. Dynamic experiments on the respiratory activity of the yeast revealed that high sodium and potassium ion concentrations adversely affected the oxygen uptake rate of the cells. Furthermore, the inhibitory effect of sodium ions on oxygen uptake was more severe. Sodium ions accumulated up to a concentration of 175 mM in the fed-batch culture mainly due to phosphate feeding. Based on these facts, a modified fed-batch culture was carried out by substituting the sodium salt with the potassium salt for phosphate which resulted in a greater cell density (112 g/l) and a higher yield of monellin (6.0 g/l).