Conidial dissemination of the rice stem rot fungus (Helminthosporium sigmoideum Cay.) in fields was most conspicuous at the end of September when observed during the season from ten days before September to the end of October. Many floating conidia were caught at the height 10 cm from the ground, while scarcely caught at 30 cm height. About one-third of conidia above mentioned had already germinated of the slide glass traps with glycerine jelly. In the conidia formation test from sclerotia, the conidiophores appeared after about 48 hours in the saturated moist chamber; at 22-25℃. After the formation of conidiophores it took about 9 and half hours until the begining of spore formation, and also it took about 10 hours from the begining to the completion of the spore formation.