Analysis of the respiro-fermentative growth of a strain of Saccharomyces cerevisiae, DSM 2155 on glucose, in a simulated 5-phase feeding strategy of fed-batch cultures executed on the Universal BIoprocess CONtrol (UBICON) system, was carried out. There was a good agreement between the estimated and the simulated values of specific growth rates. In this study, which was designed to span 0.20-0.23 h(-1) growth rates before returning to lower growth rates, the critical dilution rate at which the switch between purely oxidative and respiro-fermentative growth takes place was not observed. The biomass yield, specific substrate uptake and O-2 consumption rates as well as the consistency of the data using both carbon and available electron balances were examined. A high average value of true biomass energetic yield, eta(max)=0.707, and a low value of maintenance coefficient, m(e)=0.0114 h(-1), were obtained indicating that the organism was in no danger from the ethanol produced as a high-density fermentation with a yeast concentration above 54 g l(-1) was possible within a period of 24 h. The yeast produced also had good dough-leavening characteristics. Thus it is possible to operate a yeast plant without resorting to using respiratory quotient, which may be problematic, as the controlling parameter.