The α-ketoisovalerate decarboxylase (Kivd) is a unique lactococcal key enzyme in the decarboxylation of branched-chain α-keto acids derived from branched-chain amino acids transamination into aldehydes. These products are important aroma compounds produced during cheese ripening. In this study, the Kivd expression of Lactococcus lactis IFPL730 growing either in nitrogen-rich media or in chemically defined media supplemented with different concentrations of branched-chain amino acids and casitone was analyzed. Isoleucine starvation increased 4-fold the Kivd activity of L. lactis IFPL730 compared to cells grown in chemically defined medium with casitone as nitrogen source. Regulation of expression was at the transcription level, probably mediated by CodY, for which a consensus CodY-box sequence was identified in the kivd promoter region.