The mechanisms of AOPP herbicide resistance in two Alopecurus myosuroides biotypes were investigated. Resistant biotype Peldon Al, which is highly resistant to the phenyl-urea chlorotoluron, is moderately resistant to the AOPP herbicides diclofop-methyl, fenoxaprop-ethyl, fluazifop-P-butyl, and the CHD tralkoxydim. Resistant biotype Lines. El, which is only moderately resistant to chlorotoluron, is highly resistant to the AOPP herbicide fenoxaprop-ethyl, and moderately resistant to dielofop-methyl, fluazifop-P-butyl, and the CHD tralkoxydim. There is no clear evidence of resistance to the CHD sethoxydim in either biotype. Both Peldon Al and Lines. El exhibited moderately enhanced metabolism of diclofop-methyl. The approximate half life of diclofop was 8 and 9 HAT,respectively, compared to 17 HAT for the susceptible Rothamsted biotype. Peldon Al showed moderately enhanced metabolism of fenoxaprop-P-ethyl. However, in the highly resistant Lines. El, fenoxaprop-P-ethyl metabolism rates were intermediate between Perdon Al and the susceptible biotype. Fenoxaprop-P-ethyl metabolism in A. myosuroides was not significantly reduced by inhibitors of cytochrome P450: PBO, tetcyclasis, or ABT. While enhanced herbicide metabolism can account for the moderate AOPP/CHD resistance observed in Peldon Al in vivo, it cannot account in total for fenoxapropethyl resistance in Lines. El. Lines. El may possess one or more additional resistance mechanism. (C) 1997 Academic Press.