Extractive fermentation is shown to greatly improve the performance of Zymomonas mobilis in continuous culture during the conversion of concentrated substrates to ethanol, and it is also used to eliminate the oscillatory behavior often exhibited by Z. mobilis in conventional fermentations. An ethanol productivity of 15.6 g/Lh is achieved with the near-conversion of a 295 g/L glucose feed at a medium dilution rate of 0.11 h-1 and solvent dilution rate of 1.5 h-1. This is more than triple the productivity obtained during conventional fermentation of a 135 g/L glucose feed at the same medium dilution rate.