The fermentation characteristics of two recombinant strains of Zymomonas mobilis, viz. CP4 (pZB5) and ZM4 (pZB5), capable of converting both glucose and xylose to ethanol, have been characterized in batch and continuous culture studies. The strain ZM4 (pZB5) was found to be capable of converting a mixture of 65 g/L glucose and 65 g/L xylose to 62 g/L ethanol in 48 h with a yield of 0.46 g/g. Higher sugar concentrations resulted in incomplete xylose utilization (80 h) presumably owing to ethanol inhibition of xylose assimilation or metabolism. The fermentation results with ZM4 (pZB5) show a significant improvement over results published previously for recombinant yeasts and other bacteria capable of glucose and xylose utilization.