We have analysed the function of the open reading frame (ORF) YCL09C. The deletion of this ORF from chromosome III does not affect the physiology of the corresponding yeast strain enough to give a distinct phenotype. Nevertheless a computational analysis reveals high homology between this ORF and the enterobacterial genes encoding the regulatory subunit of acetolactate synthase. We have therefore tested the possibility that yc109cp is the regulatory subunit of yeast acetolactate synthase by in vitro enzymatic analysis. The acetolactate synthase was previously shown to be retroinhibited by its final product valine. In Escherichia coli this retro-control is assured by the regulatory subunit. Using a yeast strain carrying a complete deletion of YCL09C, we have observed the loss of such retro-inhibition. These results together with the computational predictions show that YCL09C encodes the regulatory subunit of yeast acetolactate synthase.