A new tetrasubstituted octanoic acid, named hyfraxinic acid (1), was isolated together with the known 1-deoxyviridiol (2), viridiol (3), nodulisporiviridin M (4) and demethoxyviridiol (5) from the organic extract of Hymenoscyphus fraxineus responsible for ash (Fraxinus excelsior L.) dieback in