Killer strain of Kluyveromyces lactis was constructed to prevent the aerobic deterioration of silage by disruption of KIPCK1 gene encoding phosphoenolpyruvate carboxykinase (PEPCK). The disruptants are defective in their ability of growth on lactic acid as a sole carbon source. The PEPCK activity was also deficient in the disruptants. The growth rate on lactose medium and the killing activity were equal to those of the parental strain.