A lichen-forming fungus was successfully isolated by discharged spore method from Korean lichen (Heterodermia sp.) and cultivated in pure culture. The isolate JR0012 inhibited mycelial growth of several plantpathogenic fungi. Mycelial growth of the four Pythium spp. tested was completely inhibited. Potato dextrose broth was found to be the medium favorable for largescale production of antibiotics from the isolate. Antifungal substances produced in axenic culture were partially purified. This is the first report in Korea of lichen-forming fungus successfully isolated and which exhibited strong antifungal activity against plant-pathogenic fungi, especially the four Pythium spp..