The mechanism and mode of inheritance of resistance to acetyl-coenzyme A carboxylase (ACCase)-inhibiting herbicides was investigated in a biotype of Lolium rigidum that has evolved resistance following selection with diclofop-methyl for 10 consecutive years. ACCase extracted from the resistant biotype is > 6.9 times more resistant to inhibition by diclofop than enzyme from a susceptible biotype. Similar or greater levels of resistance were found to other related herbicides. There is no difference in absorption or metabolism of diclofop-methyl or haloxyfop-methyl between the resistant and susceptible biotypes, hence differential absorption or metabolism of these herbicides does not contribute to resistance. F-1 families from reciprocal crosses between the resistant biotype and a susceptible biotype respond similarly to the herbicide and are nearly as resistant as the resistant parent, indicating that the resistance trait is nuclearly located and has incomplete dominance over susceptibility. F-2 families treated with 26 and 208 g ai ha(-1) of haloxyfop-ethoxyethyl reveal only two phenotypes: resistant plants showing no injury and susceptible plants showing no growth. At both rates of haloxyfop-ethoxyethyl, the segregation of resistance to susceptibility follows a ratio of 3:1 (R:S) that fits the predicted ratio for a single nuclear gene with high dominance. From the F-1 and F-2 data, it is concluded that resistance to haloxyfop in this resistant biotype of L. rigidum is inherited as a single nuclear incompletely dominant gene coding for a resistant form of the target enzyme ACCase.