A simple method for sporangial formation of the rice downy mildew pathogen, Sclerophthora macrospora, on infected leaf tissues was developed to facilitate diagnosis of the disease. Freshly infected young leaves showing whitish to yellowish small spots were selected and cut into small pieces about 2-3 cm in length. About 10-20 pieces were surface sterilized in a 100 ml Duran bottle with 40 ml of 70% ethanol by vigorous shaking for 30 seconds. After washing three times with distilled water, the leaf cuts were submerged in 10 ml of Milliporefiltered paddy water and incubated at 20℃ in the dark. After 8-10 h of incubation, the bottle was vigorously agitated on a voltex mixer. Aliquot amount of the suspension, 0.1-1.0 ml, was spread on a slide glass and examined under a light microscope at 50 or 100x magnification. It was found that light and 1% NaClO strongly inhibit sporangial formation of S. macrospora. Meanwhile, the use of freshly infected young leaves and washing with 70% ethanol stimulated sporangial formation of the fungus on rice leaves.