A novel metabolite, benesudon, possessing antibacterial, antifungal, cytotoxic and phytotoxic activities, was isolated from submerged cultures of the ascomycete Mollisia benesuada. Benesudon contains a reactive alpha-methyleneketone moiety which is believed to be responsible for its antibiotic activities, it reacts with cysteine and the reaction products are devoid of biological activities. Benesudon is the first secondary metabolite described from Mollisia benesuada, and its structure was determined by spectroscopic techniques.