Inumakilactone, (C18H20O8), crystallizes in space group P212121 with a = 8.15(2), b = 11.15(1), c = 17.88(2) Ǻ, Z = 4. The structure was solved by direct methods from visually estimated film data and refined to an R of 0.091 for 1338 independent reflections. The structure of the bisnorditerpenoid molecule differs from that previously proposed in respect of the orientation of the epoxy group on ring B.