Technical Abstract: Gene flow from imidazolinone (IMI)-resistant sunflower to IMI-susceptible common sunflower (Helianthus annuus L.) and prairie sunflower was studied. Both greenhouse and field studies were utilized. Greenhouse crosses made by hand between the resistant and susceptible sunflower showed that 94% of common sunflower and 79% of prairie sunflower were resistant or moderately resistant when treated with the herbicide at a rate of 40 g ai/ha. In the field, gene flow was detected up to 30 m from the pollen source for both species, and it decreased as distance from the pollen source increased. In studies conducted in 2000, 11 to 22% of the progeny were resistant at 2.5 m from the pollen source and 0.3 to 5% were resistant at 30 m from the pollen source. In studies conducted in 2001, the number of resistant progeny did not exceed 7 and 2% at 2.5 and 30 m from the pollen source, respectively. The results of this study showed that IMI-resistant domesticated sunflower outcrosses with common and prairie sunflower over distances typically encountered near production fields. Also, backcrosses of resistant hybrids with wild parents were successful, further increasing the potential for the spread of IMI-resistant feral sunflower.