Isopimpinellin (5,8-dimethoxypsoralen), C13H10O5, is a furanocoumarin isolated from the leaves and stems ofA pium australe Thouars. (Umbelliferae) collected near Concepcion, Chile. Extracts of the plant show activity against P-388 lymphocytic leukemia. Isopimpinellin is a pale yellow crystalline compound belonging to the monoclinic space groupP21/n with cell dimensionsa = 17.561(3),b = 4.3682(5),c = 17.143 A, and β = 122.97(1) °. Data were collected on an automatic diffractometer and the structure was solved by direct methods. The model was refined anisotropically to anR value of 0.046 using all 2123 measured reflections. The furanocoumarin system is nearly planar, with the lactone ring and the furan ring making angles of 1.8 ° with the central phenyl ring. The methoxyl methyl group at the heteroatom end of the molecule is out of the molecular plane whereas the otter methoxy methyl lies in the plane.