The structure and stereochemistry of caesalpinine A (l), a novel macrocyclic spermidine alkaloid isolated from Caesalpinia digyna Rottl., have been determined by means of single-crystal X-ray analysis. The high-resolution mass and NMR spectra could satisfactorily be interpreted on the basis of the X-ray structure. The compound, C25H31N3O3, is orthorhombic, with space group P212121 and unit-cell dimensions a = 24.017 (6) A, b = 19.346 (2) A, c = 9.578 (2) A, V = 4450.3 A3, and Z = 8. The crystal structure was solved by direct methods and refined by least-squares methods including anisotropic (for non-hydrogen atoms) and isotropic (for hydrogen atoms) thermal parameters to a final R value of 4.5%. Caesalpinine A possesses a 13-membered lactam ring fused to a five-membered lactam ring. It also contains two monosubstituted benzene rings.