A population of oriental mustard from Port Broughton in South Australia wasreported as not being controlled by 2,4-D. Dose response experimentsdetermined this population was resistant to both 2,4-D and MCPA, requiringgreater than 20 times more herbicide for equivalent control compared to aknown susceptible population (from Roseworthy, South Australia) and apopulation resistant only to the acetohydroxyacid synthase (AHAS)-inhibitingherbicides (from Tumby Bay, South Australia). The Port Broughton populationwas also found to be resistant to three chemical groups that inhibit AHAS;however, the level of resistance was lower than the known acetolactatesynthase–resistant population from Tumby Bay. Herbicides from other modes ofaction were able to control the Port Broughton population. Assays ofisolated AHAS from the Port Broughton population showed high levels ofresistance to the sulfonylurea and sulfonamide herbicide groups, but not tothe imidazolinone herbicides. A single nucleotide change in the AHAS genethat predicted a Pro to Ser substitution at position 197 in the protein wasidentified in the Port Broughton population. This population of orientalmustard has evolved multiple resistance to AHAS-inhibiting herbicides (AHASinhibitors) and auxinic herbicides, through a mutation in AHAS and a secondnontarget-site mechanism. Whether the same mechanism provides resistance toboth AHAS inhibitors and auxinic herbicides remains to be determined.Multiple resistance to auxinic herbicides and AHAS inhibitors in the PortBroughton population will make control of this population moredifficult.