We studied relative inhibitory activities of R(+)-quizalofop (R(+)-2-[4-(6-chloro-2quinoxalinyloxy) phenoxy]propionic acid) its ethyl ester (quizalofop-ethyl) and oxime glycol ester (propaquizafop) on the 14C-acetate incorporation into fatty acids in excised corn leaf segments and isolated corn Chloroplasts and on acetyl-CoA carboxylase (ACC) from corn Chloroplasts. Both ethyl ester (I50 4.3 x 10-5 M) and free acid (Iso 7.0 x 10-$ M) showed equally high inhibitory activity on fatty acid biosynthesis in the leaf segments. On the fatty acid biosynthesis in the corn chloroplasts however the free acid showed a stronger inhibitory efect (I50 3.6 x 1C-9 M) than the ethyl ester (Iso 4.1 x 10-7 M). In further studies on the ACC from corn chloroplasts free acid also showed more potent inhibitory activity (I50i 1.6 x 10-8 M) than ethyl ester (I50 1.1 x 10-s M) and oxime glycol ester (I50 1.0 x 1G-7 M). In an ACC assay medium the oxime glycol ester was hydrolyzed about 10% during 5-min incubation while the ethyl ester was hydrolyzed about 3% during 10-min incubation. The results indicate that a hydrolyzed product in free acid form is an actual active ingredient. In addition our previous reports and present results suggest that ACC is the primary target site of quizalofop.