Spores of many fungi in dense population, either in pastules or in suspension, germinate poorly or not at all. In some cases this is due to spore dormancy, but more commonly it is caused by substances present in or produced by spores which inhibit their own gemination. This active priciple is designated as spore germination self-inhibitor. Self-inhibitor from uredospores of oat rust fungus, Puccinia coronata f. sp. avenae, was identified as methyl cis-3,4-dimethoxycinnamate. The ED_<50> value of the cis-form of the inhibitor was 12.5pg/ml, whereas the trans-isomer had little or no activity. Irradiation of light induces isomerization from cis- to trans-form. For prevention of the isomerization methyl 3,4-dimethoxybenzalmalonate which has the symmetrical charactor was selected and synthesized. This compound expressed the same level of the activity of the natural ones. The self-inhibitors of Colletotrichum spiecies were also investigated. Their structures were demonstrated to be (E)- and (Z)-ethylidene-1,3-dihydroindol-2-one and (2S)-(3-indolyl)-propionic acid. These are the different compounds from gloeosporone which was isolated from the same strain of the fungus as its spore germination self-inhibitor.