Cyclohexanedione (sethoxydim, clethodim) and aryloxyphenoxypropionate (haloxyfop, diclofop, fluazifop, quizalofop) herbicides exhibit selective herbicidal activity on grasses. Dicots are tolerant to these herbicide classes. The effects of haloxyfop and sethoxydim on fatty acid biosynthesis in corn (Zea may L.) and pea (Pisum sativum) seedling chloroplasts were examined. The incorporation of [14C]acetate or [14C]pyruvate into fatty acids in isolated corn seedling chloroplasts was inhibited 90% or greater by 10 μM sethoxydim or 1 μM haloxyfop. The incorporation of [14C]acetate into fatty acids in isolated pea chloroplasts was not inhibited by 100 μM sethoxydim or 10 μM haloxyfop. The effect of these herbicides on fatty acid synthetase from disrupted corn chloroplasts was assayed using [14C]malonyl-CoA. Sethoxydim (10 μM) and haloxyfop (1 μM) stimulated incorporation of [14C]malonyl-CoA into fatty acids by approximately 50%. Acetyl-CoA carboxylase from disrupted corn seedling chloroplasts was inhibited by sethoxydim and haloxyfop with I50 values of 4.7 and 0.5 μM, respectively. Other aryloxyphenoxypropionate (diclofop, fluazifop, quizalofop) and cyclohexandione (clethodim) herbicides also inhibited this enzyme. Acetyl-CoA carboxylase activity from pea seedling chloroplasts was not inhibited by 1 mM sethoxydim or 0.1 mM haloxyfop. Thus, cyclohexanedione and aryloxyphenoxypropionate herbicides are potent inhibitors of acetyl-CoA carboxylase in corn, a susceptible species; whereas the enzyme from pea, a tolerant species, was tolerant to the herbicides.