In the production of ethanol by Zymomonas mobilis ATCC 10988, using sucrose as the substrate, we added immobilized invertase to the fermentation broth to promote sucrose hydrolysis. The vapid accumulation of glucose in the medium caused the inhibition of levan formation and an enhancement of the growth,th rate at the initial stage of fermentation. The formation of sorbitol could also be reduced by addition of immobilized invertase, because the accumulation of glucose and fructose was observed to be even lower than without immobilized invertase. After the addition of 0.2 g immobilized invertase (activity, 2,100 U g(-1), using 0.8 M sucrose at pH 5 and 30 degrees C) to a 1.2-1 batch fermenter containing 200 g l(-1) sucrose, the formation of byproducts was significantly reduced and the ethanol yield was increased from 0.29 to 0.40 g g(-1). High ethanol yields were also obtained from continuous cultures. When 0.2 g immobilized invertase was added to the fermenter, the dilution rate for maximal ethanol productivity was determined to be 0.2 h(-1). The coupled microbe-immobilized invertase system proved to be better than the two-stage (separate hydrolysis and fermentation) system in the yield of ethanol produced.