The mode of action of hymexazol against Pellicularia sasakii was studied. When the living mycelium of P. sasakii was incubated in aqueous solution of hymexazol for 24hr, a decrease in dry mycelial weight caused. Measurable decrease occurred at 50ppm, with progressively greater weight losses as the concentration increased. On the other hand, a decrease in the dry weight of heat-treated mycelium was negligible even at 1000ppm of hymexazol. The decrease in the dry mycelial weight was found to be closely correlated to a leakage of protein from the mycelium. At 250ppm of hymexazol, the protein leakage increased in almost linear fashion during 8hr incubation period, and then leveled off. When the mycelium was immersed in hymexazol solution for 1hr and subsequently incubated in distilled water for 23hr, a large amouut of protein leaked out from the mycelium during the 23hr incubation period in distilled water, which was in proportion to the concentration of hymexazol. While only a small amount of protein leaked out during the 1hr incubation period in hymexazol solution. Hymexazol inhibited the uptake of glucose by P. sasakii. Hymexazol was readily absorbed by the mycelium from the ambient solution within the first 1hr, and then the absorption reached to equilibrium. On potato-sucrose agar containing hymexazol at 100ppm or 10ppm, the growth of P. sasakii was reduced to some extent. At growth-inhibitory concentration of hymexazol, the hyphae appeared swollen parts as the hyphal tips and at various points along the sides of hyphae, and in certain cases the hyphae burst with loss of protoplasm.