The tolerance to alachlor [α-chloro-2′,6′?diethyl-N-(metoxy- methyl) acetanilide] of three sunflower (Helianthus annuusL.) cultivars was evaluated in an aqueous medium in a glass- house. Alachlor (0.79 - 6.32 mg L−1) caused significant reductions in the root and shoot growth of sunflower seedlings exposed to the herbicide in nutrient solution. The roots of seed- lings exposed to alachlor became progressively shorter, thicker and darker in colour as the herbicide concentration increased. Differential cultivar tolerance was demonstrated.