Acetolactate synthase (ALS) is the site of action of several new, structurally diverse classes of herbicides (sulphonylurea1–4, imidazolinone5,6 and triazolo pyrimidine or sulphonanilide7,8). These herbicides are unusual inhibitors in that they bear no obvious similarity to substrates (pyruvate and α-ketobutyrate), cofactors (thiamine pyrophosphate, FAD and magnesium), or allosteric effectors (valine, leucine and isoleucine) of this enzyme. They also interact in a complex way, in that time-dependent inhibition is observed1,6,8,9, with tightest binding of the herbicide occurring under conditions of enzymatic turnover1,9. Recently, the identity of the herbicide-specific site has been suggested by the discovery that the sequence of pyruvate oxidase10 is very similar to that of ALS11–16. We propose that the herbicide-specific site of ALS is an evolutionary vestige of the quinone17 binding site of pyruvate oxidase. Consistent with this proposal, the ubiquinone homologues Q0and Q1 are potent inhibitors of ALS, and Q0, an imidazolinone herbicide (imazaquin), and a sulphonanilide herbicide, each compete with a radiolabelled sulphonylurea herbicide (sulphometuron methyl) for a common binding site on ALS.