The influence of mixing (in the range from 300 to 1100 rpm) on the performance of Zymomonas mobilis anaerobic continuous culture was studied. Biomass yield and ethanol productivity were improved at higher stirring intensities, in parallel with a decrease in byproduct synthesis. A mixing-dependent formation of minor amounts of fructose was first demonstrated in Z. mobilis anaerobic culture on glucose. The rate of fructose synthesis and the specific activity of the key enzyme in the pathway to fructose, phosphoglucose isomerase (EC 5.3.1.9), had a maximum at 700 rpm. (C) 2003 Elsevier Science Ltd. All rights reserved.