Cornexistin, a new compound demonstrating promising herbicidal activity, was purified from the culture filtrate of a newly-isolated fungus identified as Paecilomyces variotii SANK 21086. The compound was extracted with organic solvents from the culture filtrate, purified using column chromatography on Sephadex LH-20 and finally crystallized from methylene chloride. Following analysis of its physico-chemical properties it was identified to be a new compound belonging to the nonadride group. Chemical structure elucidation was conducted by analyses of various spectral data and the structure was finally confirmed by means of X-ray crystallographic analysis. Based on its herbicidal characteristics cornexistin may be classified as a postemergence herbicide active against certain young annual and perennial monocotyledonous and dicotyledonous plants with selective protection for corn.