Lolium rigidum biotype SR4/84 is resistant to the herbicides diclofop-methyl and chlorsulfuron when grown in the field, in pots, and in hydroponics. Similar extractable activities and affinities for acetyl-coenzyme A of carboxylase (ACCase), an enzyme inhibited by diclofop-methyl, were found for susceptible and resistant L. rigidum. ACCase activity from both biotypes was inhibited by diclofop-methyl, diclofop acid, haloxyfop acid, fluazifop acid, sethoxydim, and tralkoxydim but not by chlorsulfuron or trifluralin. Exposure of plants to diclofop-methyl did not induce any changes in either the extractable activities or the herbicide inhibition kinetics of ACCase. It is concluded that, in contrast to diclofop resistance in L. multiflorum and diclofop tolerance in many dicots, the basis of resistance to diclofop-methyl and to other aryloxyphenoxypropionate and cyclohexanedione herbicides in L. rigidum is not due to the altered inhibition characteristics or expression of the enzyme ACCase. The extractable activities and substrate affinity of acetolactate synthase (ALS), an enzyme inhibited by chlorsulfuron, from susceptible and resistant biotypes of L. rigidum were similar. ALS from susceptible and resistant plants was equally inhibited by chlorsulfuron. Prior exposure of plants to 100 millimolar chlorsulfuron did not affect the inhibition kinetics. It is concluded that resistance to chlorsulfuron is not caused by alterations in either the expression or inhibition characteristics of ALS.