The crystal structure of iso-eremolactone, C₂₀H₂₆O₂, a plant product derivative from a new biogenetic series, has been solved by the direct methods of Karle & Hauptman for non-centrosymmetric structures. The space group is P2₁2₁2₁, and a= 12.866, b=20.200, c=6.685 Å. The final R value is 0.063. Long sp³–sp³ carbon-carbon bonds in the structure are associated with a high degree of substitution at the carbon atoms, and also with bond angles which differ considerably from the ideal tetrahedral values. The thermal motion in the structure has a substantial internal component, which results from the lability of bond angles with non-ideal values.