Residual starch from flour wet milling, supplemented with crushed wheat grains, was hydrolysed and evaluated for ethanol production by Zymomonas mobilis ZM4 and an industrial ethanol-producing strain of Saccharomyces cerevisiae. The feasibility of replacing yeast extract with ammonium sulphate or corn steep liquor as a nitrogen source in the hydrolysed waste starch (HWS) media was confirmed. With glucose concentrations of 80-110 g L-1, Z. mobilis ZM4 demonstrated superior fermentation kinetics in all three media, with higher specific rates of glucose uptake and ethanol production, greater productivities and ethanol yields close to theoretical. In a repeated batch process (five cycles), rapid cell concentration was achieved between cycles (via cell settling) with the flocculent strain Z. mobilis ZM401. This resulted in very significant productivity increases and illustrated the potential use of this strain for improved technology and cost reductions in larger scale processes. (c) 2006 Elsevier Ltd. All rights reserved.