Pycnospores of Mycosphaerella pinodes, a non-obligate parasite of pea plant did not induce pisatin accumulation until they established infection, probably suppressing defense reaction of the host. This fungus was highly tolerant to pisatin as assessed by the inhibitory activity to spore germination or germ-tube elongation (ED₅₀=500ppm). The fungus, however, could not establish infection on pea if a very low concentration (less than 50ppm) of pisatin was administered artificially into the spore inoculum. The perforation of cellophane-sheet by germ-tube of M. pinodes was also inhibited by the same concentration of pisatin that inhibited infection on the host. These results suggest strongly that the importance of phytoalexin as an anti-infectional factor in defense reaction of host rather than antifungal substance and pea pathogen, M. pinodes suppresses pisatin production in early stage of infection in order to avoid its inhibitory effect on infection.