In eight experiments the competition between sun and shade plants was studied in different light environments. In higher light intensities the competitive ability of sun species was definitely greater. In lower light intensities, competitive ability did not differ basically between sun and shade species, but seemed mainly to depend on the weight of the plant at the start of the expiment. It is concluded that the competitive ability of sun and shade plants does indeed correlate positively with the light intensity of their natural habitats. The effect of the red/far-red ratio on competitive ability cannot be predicted from the experiments, but it is probable that when competing, sun species will be disadvantaged by a low red/far-red ratio.