A growth inhibitor with the molecular formula C10H10O4Cl2 was isolated from a medium which had supported growth of a species of Cryptosporiopsis, an imperfect fungus. The compound was named cryptosporiopsin and was characterized on the basis of its physical and chemical properties. Cryptosporiopsin was active in vitro against many wood-rotting basidiomycetes as well as some phycomycetes, ascomycetes, and deuteromycetes. It was more effective than Dithane M-22 or nystatin against the in vitro spore germination of Phytophthora infestans. It was active to varying degrees against six species of bacteria. Cryptosporiopsin was compared with Orbenin and streptomycin against a penicillin-resistant strain of Staphylococcus aureus.