Sorbitol is formed as the major by-product in ethanol fermentations by Zymomonas mobilis when both glucose and fructose are present in the fermentation medium. The amount of sorbitol produced was equivalent to as much as 11% of the original carbon source, decreasing the ethanol yield correspondingly. Only minor amounts of sorbitol were formed from glucose or fructose alone. The formation of sorbitol is apparently a consequence of the inhibition of fructokinase by glucose.