Acetolactate synthase (ALS) catalyzes the first common step in the biosynthesis of branched-chain amino acids in plants and is the target of several herbicides. ALS inhibitors have enjoyed popularity as herbicides due to numerous attributes, although their current adequacy in weed control programs is hampered by herbicide resistance. Most cases of ALS-inhibitor resistance have resulted from selection of an altered target site. The study herein reports on an alanine by threonine amino acid substitution at position 122 of ALS as the basis for imidazolinone-specific resistance in an A. hybridus population from Illinois. In vitro inhibition of enzymatic activity (I(50)) required 1000-fold greater concentration of imazethapyr in the resistant population compared with a susceptible control. This mutation represents the second ALS alteration associated with herbicide resistance in a natural A. hybridus population.