The effect of light on conidial development of a nonphotoinduced isolate of Helminthosporium oryzae was investigated. Near-ultraviolet light was not required for conidiophore induction, but conidial development was suppressed by blue light applied at a definite stage of conidiophore development, and the conidiophores were dedifferentiated into longer, slender aerial hyphae. Furthermore, the effect of blue light could be reversed by immediate exposure to near-ultraviolet light and the effects of these two spectral regions were alternatively reversible.Thus, it is concluded that a photosensitive system, called mycochrome, is involved in the conidial development of this fungus.