A bicyclic sesterterpene from Drechslera siccans was purified and structurally characterized. During the isolation procedure, the toxic component was monitored by the assay using Italian ryegrass (Lolium multiflorum Lam.), one of the host plants of the pathogen. Siccanol (1) named arbitrarily was shown to have a molecular formula of C_(25)H_(38)O₄. Siccanol completely inhibited the root growth of the Italian ryegrass seedlings at a level of 100 ppm.