When grown in a minimal-defined medium, a strain of Drechslera siccans, a pathogenic fungus isolated from seeds of Lolium perenne, produced phytotoxic metabolites. This strain is one of the best toxin producers among several grass pathogenic fungal strains collected and tested to find phytotoxins to be used as natural herbicides of monocot weeds. From the culture filtrates of D. siccans, we isolated a new phytotoxic trisubstituted naphthofuroazepinone, named drazepinone, and characterised it as a 3,5,12a-trimethyl-2,5,5a,12a-tetrahydro-1H-naphtho[2',3':4,5]furo[2,3-b]azepin-2-one. Assayed at 2 microg microl(-1) solution the novel metabolite proved to have broad-spectrum herbicidal properties, without antibacterial and antifungal activities, and low zootoxic activity. Its original chemical structure and the interesting biological properties make drazepinone a potential natural herbicide.