Effects of near-ultraviolet irradiation on the sporulation of Pyricularia oryzae Cavara were investigated by daily count of conidial formation of the fungus grown at 27±1C on potato sucrose agar inoculated uniformly flooded with fungal suspension. Continuous irradiation with near-UV (366nm) on 4 isolates of P. oryzae generally shortened the time required to reach maximum sporulation, but the effect of continuous irradiation on the amounts of sporulation varied among isolates. Irradiation for the first 3 days resulted the highest sporulation and accelerated conidiophore formation. Sporulation, however, was less in continuous darkness or in darkness for the first 3 days. Near-UV absorbing, water-soluble and heat-unstable substances with maximum absorption at 325nm were detected from the cultures of an isolate irradiated for 4 days, but not from unexposed cultures. When the irradiated fungal extracts were added to medium, sporulation under irradiation increased remarkably. In darkness, both fresh and autoclaved fungal extracts depressed sporulation.