Herbicides that inhibit acetolactate synthase (ALS) have been important weed management tools for nearly 20 years. In recent years, resistance to ALS inhibiting herbicides has increased. In 1996, a population of common sunflower near Howard, South Dakota was suspected to be resistant to chlorimuron and imazethapyr, both ALS-inhibiting herbicides. Whole-plant ALS enzyme assays and herbicide dose response experiments concluded that cross resistance between chlorimuron and imazethapyr was present in the Howard common sunflower population. The percentages of resistant to sensitive individuals within the resistant population also indicated that the resistant population was not homogeneously resistant to either herbicide. Herbicide penetration and translocation experiments showed that resistant plants absorbed approximately 44% and 36% less 14C-chlorimuron and 14C-imazethapyr, respectively, compared to sensitive plants. Translocation of 14C did not vary. An alanine to valine substitution at amino acid position 204 of the ALS gene was found in six of seven clones from resistant plants. A frame shift occurred in Region B of the ALS gene, suggesting that multiple copies exist in the genome. Gene flow experiments suggested that resistance is due to a semi-dominant, nuclear-encoded ALS gene.