Fermentation of starch hydrolysates {wet-milling corn process} exhibited a lower conversion efficiency in the presence of yeast fermentation backset compared to fourfold concentrated Zymomonas mobtiis fermentation backset. In dry milling, Z. mobilis fermentation backset stimulated ethanol production in the case of liquefied corn feed stocks but retarded fermentation with liquefied milo feedstocks. By using an inoculum cascade method, this retardation was found to be due to nutrient deficiencies in the milo flour. After adjustment of this deficiency, Z. mobilis fermentation in the presence of backset was equal to or better than that obtained in the absence of backset.