<P>Three xylose-fermenting recombinant Zymomonas mobilis strains containing different Peno-talB/tktA operon terminators were engineered. Each showed similar levels of foreign protein expression and xylose fermentation performance. Strain CP4-P2-1 was further used to compare the glucose/xylose co-fermentation under various cultivation environments to improve the efficiency of the process. Optimal co-fermentation was achieved at 30-34 °C and pH 5.5 using xylose-grown preculture cells giving 20.5 g ethanol/l, ethanol productivity of 0.43 g/l h and ethanol yield of 0.44 g/g at 48 h. Adverse culture conditions mainly influenced the efficiency of xylose fermentation but not glucose fermentation. The key factors affecting co-fermentation were also explored at the molecular level. This study provides valuable insights into the effective harnessing of biomass resources.</P>