Zymomonas mobilis is a potential candidate for fuel ethanol production because of its high ethanol productivity. A key enzyme in ethanol production is alcohol dehydrogenase (ADH). Z, mobilis possesses two isozymes, ZADH-1 and ZADH-2. To clarify their physiological roles, mutants with modified ADH were isolated by selection based on resistance to allyl alcohol. From the physiological characteristics of a ZADH-2-negative mutant, it is suggested that ZADH-1 functions as the major ADH, while ZADH-2 could become functional in the latter stages of fermentation.