Technical Abstract: The antagonism between by yeast and lactobacilli is largely dependent on the initial population of each organism. While homo-fermentative lactobacillus present higher inhibitory effect upon yeast when in equal cell number, in industrial fuel ethanol conditions where high yeast cell densities prevail hetero-fermentative lactobacillus are more deleterious, since they succeed in competing with yeast for sugars during fermentation. Both bacteria metabolic types caused reduced ethanol yield during yeast fermentation, but this effect was more pronounced with the hetero-fermentative strain in industrial fuel ethanol conditions. The greater glycerol formation by yeast as well as higher concentrations of bacterial metabolites produced (lactic and acetic acids, plus mannitol) and higher bacterial growth, all led to a greater sugar consumption and decreased ethanol yield in industrial yeast fermentation contaminated with the hetero-fermentative lactobacillus strain. Studies are in progress on the prevalence of homoand hetero-fermentative lactobacilli in Brazilian fuel ethanol plants and on the differential effect of these two types of bacteria on glycerol production by yeast.