Suspected 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.