The effects of 3-hydroxy-β-ionone on the growth of hypocotyls of dark-grown dwarl bean (Phaseolus vulgaris L. cv. Morocco) and tall bean (cv. Kentucky Wonder) were examined. 3-Hydroxy-β-ionone inhibited growth of the hypocotyl of both cultivars at doses higher than 3 ng per plant. Seedlings of the tall cultivar responded to a slightly lesser extent to 3-hydroxy-β-ionone than those of the dwarf cultivar. Changes in the levels of 3-hydroxy-β-ionone in hypocotyls of both cultivars 24 h after the onset of light irradiation, were determined by high-performance liquid chromatography and the results compared with the results of light-induced inhibition of growth. The difference in levels of 3-hydroxy-β-ionone between the two cultivars was considerable, and the changes in its levels in the hypocotyls of each cultivar reflected the light-induced inhibition of growth of the hypocotyls of the respective cultivars. These results suggest that the endogenous growth inhibitor. 3-hydroxy-β-ionone, may play an important role in the inhibition by light of hypocotyl growth of the two cultivars and may serve to distinguish the growth habits of these cultivars.