Most people are familiar with the sight on a young seedling bendingtowards a window or the brightest source of light to which it is exposed. Thisdirectional growth response is known as phototropism. Went (1928) proposedthat phototropism is caused by a lateral gradient of growth-promoting auxin inthe bending organ. This led to the Cholodny-Went theory of photo-and geotropiccurvatures(1937). In contrast to the C.-W. theory， Bruinsma recently found thatthe shaded half did not contain more auxin than the illuminated ones. Instead itwas found that the even distribution of auxin was accompanied by a lateralgradient of growth-inhibitory substances during phototropic curvature. We have isolated a light-induced growth inhibitory substance related tophototropism， p-hydroxy cinnamic acid， from light-grown bean (Vigna mungoL.) seedlings. The light-grown seedlings were observed to contain a higher levelof p-hydroxy cinnamic acid than the dark control determined by measurement ofthe peak area on HPLC.