In this study, we have used ultraviolet (UV) and gamma-ray induction to get a catabolite repression resistant and thermotolerant mutant with enhanced ethanol production along with optimization of sugar concentration and temperature of fermentation. Classical mutagenesis in two consecutive cycles of UV- and gamma-ray-induced mutations evolved one best catabolite-resistant and thermotolerant mutant Saccharomyces cerevisiae MLD10 which showed improved ethanol yield (0.48 +/- 0.02 g g(-1)), theoretical yield (93 +/- 3 %), and extracellular invertase productivity (1,430 +/- 50 IU l(-1) h(-1)), respectively, when fermenting 180 g sugars l(-1) in molasses medium at 43 A degrees C in 300 m(3) working volume fermenter. Ethanol production was highly dependent on invertase production. Enthalpy (Delta H*) (32.27 kJ M-1) and entropy (Delta S*) (-202.88 J M-1 K-1) values at 43 A degrees C by the mutant MLD10 were significantly lower than those of beta-glucosidase production by a thermophilic mutant derivative of Thermomyces lanuginosus. These results confirmed the enhanced production of ethanol and invertase by this mutant derivative. These studies proved that mutant was significantly improved for ethanol production and was thermostable in nature. Lower fermentation time for ethanol production and maintenance of ethanol production rates (3.1 g l(-1) h(-1)) at higher temperature (43 A degrees C) by this mutant could decrease the overall cost of fermentation process and increase the quality of ethanol production.