The molecular geometry of caesalpinine C (1) (celallocinnine), a macrocyclic spermidine alkaloid isolated from Caesalpinia digyna Rottl., has been determined by means of single-crystal X-ray analysis. The 1H and 13C n.m.r. spectra could satisfactorily be interpreted on the basis of the X-ray structure. The compound, C25H31N 3O2, is orthorhombic with space group P212 121 and lattice constants a = 9.963(2), b = 8.026(2), c = 28.627(5) Å, cell volume V = 2 289.1 Å3, and Z = 4. Diffractometer intensity measurements were made with Cu-Kα radiation and least-squares adjustment of the atomic parameters converged to a final R value of 0.05.