Acetohydroxyacid synthase (AHAS) is required for the biosynthesis of two acetohydroxyacids: acetolactate and ace-tohydroxybutyrate. AHAS occupies the same central role in valine, leucine, and isoleucine biosynthesis in plants as it does in microbes. Feedback of all three amino acids inhibits the enzyme, but a cooperative inhibition is observed when both valine and leucine are present. The structural requirements for inhibition of AHAS by the imidazolinones correlate, to a certain degree, with the structural requirements for herbicidal activity. AHAS from all species was highly sensitive to inhibition by imazapyr and imazethapyr, but, imazethapyr was a stronger inhibitor than imazapyr. AHAS activity from pea, lima bean, and soybean appeared to be the most tolerant to the herbicides. Microorganisms may possess as many as six isozymes of AHAS with different sensitivities to feedback inhibition by amino acids. Similarly, bacterial AHAS isozymes differ in their sensitivities to sulfonylureas.