In studies of the fungus, Alternaria tomato , we found the blue and near ultraviolet reversible photoreaction, which plays an important role in photocontrol of conidial development, in the light-minus-dark difference spectrum of the intracellular particulate fraction isolated from dark-grown mycelia. That is, after irradiation with near ultraviolet, the difference spectrum of the particulate ... [Show full abstract] fraction showed a dip at 300 nm and a peak at 400 nm. When irradiated with blue light after exposure to near ultraviolet, both the peak at 400 nm and the dip at 300 nm partially disappeared. This change was reversibly repeated by alternating doses of blue and near ultraviolet. Action spectra for the light-mediated changes in the difference spectrum showed the peaks at 300 nm and 410 nm.