In an attempted synthesis of the alkaloid matrine, a compound C20H33N3, of unknown structure, was isolated. Crystals of its methiodide derivative from methanol, C20H33N3,l,xCH3OH (x=ca.½), are orthorhombic, a= 8·01, b= 17·70, c= 31·3 A, Z= 8, space-group Pbca. The structure was determined with Mo-Kαscintillation-counter data by Patterson and Fourier methods, and refined by least-squares, the final R being 0·17 for 1876 observed reflexions. The complete molecular structure has been established.