The effects of ultrasonic irradiation during milk fermentation have been investigated in terms of the cell viability, beta-galactosidase activity, the pH value of the culture medium, the degree of lactose hydrolysis and glucose content. The results showed that the ultrasonic irradiation caused the intracellular beta-galactosidase to be released from the lactic acid bacteria cells. The released beta-galactosidase showed a higher lactose hydrolysis activity than that in the cells. The beta-galactosidase that was released to the medium has been more effectively used in pH-controlled fermentation. The results also showed that the continuous sonication caused the cell viability to decrease, but the viable cell count was again increased with static incubation after sonication. High degrees of lactose hydrolysis and high cell viabilities were obtained with the combination of pH-controlled sonicated fermentation and static incubation.