Ethanol production by Zymomonas mobilis (ATCC 10988) was studied by using different concentrations offglucose (50-300 g/litre). A glucose concentration of 50-150 g/litre showed the highest ethanol-yield efficiency. Step-feeding of glucose was attempted to increase ethanol production and to reduce substrate inhibition. A step-feeding system for a total glucose concentration of 200 g/litre was found suitable. The specific growth rate was reduced as the number of steps increased. The effect of inoculum age, yeast extract, and peptone on the step-feeding system was studied. A suitable combination of yeast extract and peptone was selected on the basis of the central-composite-rotatable-design (CCRD) procedure.