Ovatifolin, C17H22O5, is a sesquiterpene lactone isolated from the aerial parts ofPodanthus mitiqui Lindl. (Compositae) collected near Pichidangui, Chile. Extracts of the plant exhibit cytotoxic activity against KB cell cultures. The pale yellow crystals belong to the orthorhombic space groupP212121 with cell dimensionsa = 8.159(1),b = 6.640(1), andc = 29.314(4) A. Data were collected on an automatic diffractometer and the structure was solved by direct methods. The model was refined anisotropically to anR value of 0.047 using all 1742 measured intensities. The 10-memberedtrans-trans-germacradiene ring adopts a chair-chair conformation with C(4) and C(10) methyl groups orientedsyn on the β face of the molecule. The conformation leads to a crossed double-bond system in which transannular interactions shift the ultraviolet absorption to 210 nm. The five-membered lactone ring adopts a distorted chair conformation and istrans-fused to the 10-membered ring.