bacillus and lactobacillus are bacteria that usually contaminate the ethanolic fermentation by yeasts and may influence yeast viability. as microorganisms can be killed by ionizing radiation, the efficacy of gamma radiation in reducing the population of certain contaminating bacteria from sugarcane must was examined and, as a consequence, the beneficial effect of lethal doses of radiation on some parameters of yeast-based ethanolic fermentation was verified. must from sugarcane juice was inoculated with bacteria of the genera bacillus and lactobacillus. the contaminated must was irradiated with 2.0, 4.0, 6.0, 8.0 and 10.0 kgy of gamma radiation. after ethanolic fermentation by the yeast (saccharomyces cerevisiae) the total and volatile acidity produced during the process were evaluated; yeast viability and ethanol yield were also recorded. treatments of gamma radiation reduced the population of the contaminating bacteria in the sugarcane must. the acidity produced during the fermentation decreased as the dose rate of radiation increased. conversely, the yeast viability increased as the dose rate of radiation increased. gamma irradiation was an efficient treatment to decontaminate the must and improved its parameters related to ethanolic fermentation, including ethanol yield, which increased 1.9%.