Aleurodiscal, an antifungal antibiotic has been isolated from mycelial cultures of Aleurodiscus mirabilis (Berk. & Curt.) Höhn. It causes abnormal apical branchings of Mucor miehei hyphae at very low concentrations. The structure and absolute configuration of the antibiotic has been determined by a single crystal X-ray analysis and hydrolysis to D-(+)-xylose. Aleurodiscal is a hydroxysesterterpene aldehyde β-D-xyloside with a novel carbon skeleton.