A novel labdane diterpene alkaloid, vitexlactam A (1) was isolated as a prism from the n-hexane extract of the fruits of Vitex agnus-castus through normal and reverse phase column chromatography. Its structure was elucidated to be 6beta-acetoxy-9alpha-hydroxy-13(14)-labden-16,15-amide, based on chemical and spectral evidences including 1D and 2D NMR spectra. The structure was confirmed by X-ray crystallographic analysis. Compound 1 is the first naturally occurring labdane diterpenoid containing an alpha,beta-unsaturated gamma-lactam moiety. (C) 2002 Elsevier Science Ltd. All rights reserved.