Yeast ethanol fermentations contain contaminating bacteria and yeast, with Lactobacilli being a frequent contaminant. These bacteria tolerate the low pH and high ethanol concentrations present in the fermentation, and can decrease the ethanol yield. Fermentations are routinely treated with antibiotics to reduce bacterial contaminants; however, this does not always resolve the contamination, perhaps due to biofilm formation by the contaminating bacteria. In this study, a novel inhibitor of Lactobacillus sp. biofilm and planktonic growth was analyzed. This inhibitor does not inhibit growth of any other bacteria or yeast tested, making it the first known Lactobacillus biofilm inhibitor that is specific for Lactobacillus. This inhibitor was added to a yeast catalyzed ethanol fermentation of corn mash experimentally infected with Lactobacillus fermentum. The addition of the inhibitor increased the ethanol yield and decreased the residual glucose to the levels seen in an uninfected control. This inhibitor is a good candidate for prevention of Lactobacillus contamination in ethanol fermentations on a commercial scale.