In the course of screening for novel naturally occurring insecticides from Chinese crude drugs, an MeOH extract of Alpinia oxyphylla was found to possess insecticidal activity against larvae of Drosophila melanogaster Meigen. From the extract, an insecticidal compound was isolated by bioassay-guided fractionation and identified as nootkatone (1) by GC, GC-MS, and (1)H and (13)C NMR spectroscopy. In bioassays for insecticidal activity, 1 showed an LC(50) value of 11.5 micromol/mL of diet against larvae of D. melanogaster and an LD(50) value of 96 microg/adult against adults. Epinootkatol (1A), however, showed slight insecticidal activity in both assays, indicating that the carbonyl group at the 2-position in 1 was the important function for enhanced activity of 1.