lntracellular pH (pH;) was measured on-line in a bioreactor using a fluorescent pHi indicator, 9-aminoacridine, and controlled fed-batch cultivations of yeast cells based on pHj (FB-pH;) were performed. In FB-pHi cultivations, automated glucose additions were made to the culture in response to culture pHi. The average ethanol (anaerobic product) yield was significantly lower l0.12 g g-' glucose) in fed-batch pHj cultivations with 100 pprn glucose additions (FB-pH,-IOO cultivation) vs. 0.48 g g-' glucose in batch] and cell yield was higher (0.54 g g-' glucose in FB-pHj-100 cultivation vs. 0.3 g g-' glucose in batch) compared to batch cultivation. An expression has been derived to calculate changes in pHi from measured fluorescence values when the cell concentration increases during growth. Cultivations based on pHi, performed with different magnitudes of glucose addition (100, 50, and 10 pprn additions), showed that lower magnitudes of glucose addition resulted in lower ethanol yields while cell yield remained unaffected. The ratio of specific oxygen uptake rate to specific glucose uptake rate (OUR/GUR) increased with decrease in magnitude of glucose additions in FB-pHj cultivations, suggesting that the culture aerobic state was higher when the magnitude of glucose addition was lower. The average cell productivity in FB-pH; cultivations was 29% higher than in batch cultivation. Cells were also cultivated at high OUR conditions, and the results are compared with other cultivations.