Lactococcus lactisFI9078, a construct carrying a disruption of theldh gene, converted approximately 90% of glucose into lactic acid, like the parental strain MG1363. This unexpected lactate dehydrogenase activity was puri-fied, andldhB was identified as the gene encoding this protein. The activa-tion of ldhB was explained by the insertion of an IS905-like element that created a hybrid promoter in the intergenic region upstream ofldhB.