tFermentation redox potential was monitored during cell-recycling continuous ethanol operation. Thecell-recycling system (CRS) was operated using two hollow fibre (HF) membranes (pore sizes 0.20 and0.65 m) at three dilution rates (0.02, 0.04 and 0.08 h−1). Saccharomyces cerevisiae NP 01 were recycled inthe fermenter at a recycle ratio of 0.625. Aeration was provided at 2.5 vvm for the first 4 h and then furthersupplied continuously at 0.25 vvm. As steady state was established, results showed that the fermentationredox potential was lower for processes employing CRS than those without. At the same dilution rates,the sugar utilization and ethanol production with CRS were higher than those without CRS. The highestfermentation efficiency (87.94 g/l of ethanol, ∼90% of theoretical yield) was achieved using a 0.2-mHF membrane CRS at a dilution rate of 0.02 h−1. It was found that 7.53–10.07% of the carbon derivedfrom glucose was incorporated into the yeast. Further, at the same dilution rates, yeast in the processeswith CRS incorporated less carbon into ethanol than in those grown without CRS. This result suggeststhat processes involving CRS utilize more carbon for metabolite synthesis than biomass formation. Thisindicated that the processes with CRS could utilize more carbon for metabolite synthesis than biomassformation.