A batch fermentation utilizing Saccharomyces cerevisiae BY4742 was conducted to determine the inhibitory effects of highly concentrated substrate and product levels on yeast. Experiments were performed to determine the largest dosage of substrate and the largest product concentration that the yeast could tolerate in a very high gravity fermentation process. The yeasts growth and fermentation activities were characterized by changes in the biomass and ethanol yield under different substrate and product concentrations during fermentation. All of the experiments were performed at a pH of 5.0 and a temperature of 35 degrees C with a stirring rate of 180 r/min and a fermentation time of 96 h. Furthermore, five cycles of acclimatization were conducted to improve the yeasts tolerance to ethanol. Ethanol yield was maximized at 95% with a product concentration of 39 g/L and substrate dosage of 80 g/L. The system exhibited an obvious increase in cell growth and ethanol production with increasing substrate dosage up to a critical point of 160 g/L glucose (53 g/L ethanol fermented and an ethanol yield of 65%). Above this point, cell growth and ethanol production were inhibited with the final product concentration increasing only slightly with an increase in the initial substrate concentration. The end product (ethanol) was shown to be the primary factor inhibiting yeast growth and fermentation activity because the yeast would completely stop growing and fermenting when the initial exogenous ethanol concentration exceeded 70 g/L. The endogenous ethanol exerted a greater impact on yeast performance during anaerobic fermentation than exogenous ethanol. Five cycles of acclimatization significantly improved the yeast density, cell morphology, and ethanol production during very high gravity fermentation. The ethanol yield increased from 6% to 30% under an initial exogenous ethanol concentration of 60 g/L.