Low energy ultrasound irradiation (20 kHz, 1.0 W·L^-1) was applied to enhance bioethanol production from whey ultrafiltration permeate by co-immobilized Saccharomyces cerevisiae cells and β-galactosidese enzyme. Sugar utilization and ethanol formation were investigated as a function of hydraulic retention time (HRT) between 12 and 36 h. Maximum ethanol production under HRT of 36 h was 26.30 g·L^-1 with ultrasound exposure, and 23.60 g·L^-1 without. Maximum ethanol yield was 0.532 g·g^-1 lactose in the fermentation process with ultrasound irradiation, and 0.511 g·g^-1 without. For the continuously operating bioreactors, the maximum rates of sugar utilization were 98.9 and 92.4% for the yeast with and without ultrasound exposure, respectively. These results highlight the positive effect of low-intensity ultrasounds in bioethanol fermentation from whey permeates, raising new perspectives for its disposal.