The biochemical basis for diclofop resistance in an Italian ryegrass (Lolium multiflorum Lam.) biotype discovered in Oregon was examined. Herbicide rates that inhibited shoot growth by 50% (GR50 values) were determined for two aryloxyphenoxypropionic acid herbicides (diclofop, haloxyfop) and one cyclohexanedione herbicide (sethoxydim). As compared to a wild type Italian ryegrass biotype, the GR50 values for diclofop, haloxyfop, and sethoxydim were approximately 130-, 22-, and 2-fold greater, respectively, for the resistant biotype. There were little or no differences in the retention, absorption, translocation, or metabolism of diclofop-methyl in resistant and susceptible biotypes. The susceptibility of acetyl-CoA carboxylase (ACCase) to inhibition by selected graminicide herbicides was evaluated in extracts from etiolated shoots of both resistant and susceptible biotypes. The herbicide concentrations that inhibited ACCase activity by 50% (I50 vulues) for diclofop, haloxyfop, and quizalofop were approximately 28-, 9-, and 10-fold greater, respectively, for the enzyme from the resistant biotype. For the cyclohexanedione herbicides, sethoxydim and clethodim, the I50 values for ACCase were similar for both biotypes. It is concluded that resistance to diclofop and other aryloxyphenoxypropionic acid herbicides in the Italian ryegrass biotype from Oregon is due to the presence of a tolerant form of ACCase. This modification confers tolerance to the aryloxyphenoxypropionic acids but little or no tolerance to the cyclohexanediones.