AbstractSuspected imazaquin‐resistant accessions of Amaranthus palmeri were studied to determine the magnitude of resistance and cross‐resistance to acetolactate synthase (ALS)‐inhibiting herbicides and compare differential tolerance of A palmeri and Amaranthus hybridus to ALS inhibitors. Five of seven A palmeri accessions were resistant to imazaquin. The most imazaquin‐resistant accession, accession 7, also showed 74, 39 and 117 times higher resistance than the susceptible biotype to chlorimuron, diclosulam and pyrithiobac, respectively. Resistance to imazaquin and cross‐resistance to other ALS inhibitors in A palmeri was due to a less‐sensitive ALS enzyme. A palmeri was 70 times more tolerant to imazaquin than A hybridus. A palmeri was also seven times more tolerant to pyrithiobac than A hybridus. Differences in ALS enzyme sensitivity could not fully account for the high tolerance of A palmeri to imazaquin compared to A hybridus. Both species were equally affected by chlorimuron and diclosulam.© 2001 Society of Chemical Industry