This study isolated and identified oxindole-3-acetic acid methylester from the flowers (buds) of several Oenothera species, representing the first report of an alkaloid in this genus. Air-dried plant material was extracted with 90% MeOH, followed by acid-base partitioning and preparative HPLC for isolation. The compound was identified via comparison of MS, 1H NMR, 13C NMR data, and melting point with published values. Oxindole-3-acetic acid methylester was only detected in flowers and buds, not in stems or leaves, and exhibited an indole-like odor, possibly contributing to the flowers' unpleasant amine-like smell. Ecologically, Oenothera species are pollinated by night-flying, long-tongued moths, and caterpillars of Hyles lineata (a common moth in Colorado) fed on the flowers but did not accumulate the alkaloid in their tissues.