Broadleaf weeds cause considerable yield losses to sunflower production in all regions of the world. Resistance to the imidazolinone herbicides, imazethapyr and imazamox, found in a population of wild sunflower, could have great value for controlling many broadleaf weeds. The herbicide resistance was successfully transferred from resistant wild sunflower plants to a cultivated sunflower inbred line, HA 425. The objective of this investigation was to determine the inheritance of resistance to the herbicide imazamox in HA 425. Segregation ratios of plants in F2 and testcross populations indicated that resistance was controlled by two genes, a major gene having a semi-dominant type of gene action (Imr1), and a second gene (Imr2) with a modifier effect when the major gene is present. Resistance in sunflower can only be achieved with homozygocity (Imr1, Imr1, Imr2, Imr2) of both resistance genes in an inbred line or in a hybrid. Completely resistant hybrids require having resist ance factors in both parents.