Cucurbitine is a natural amino acid newly found as the effective component in the crude drug, Cucurbita moschata Duch, which inhibits the growth of immature Schisto soma japonicum. This paper describes the X-ray diffraction studies on the structure of the perchlorate of the amino acid. Preliminary examinations show that the crystal belongs to the monoclinic system with space group P21 and unit cell dimensions a = 5.68 ± 0.01?, b = 12.78 ± 0.01?, c = 6.23 ± 0.01?, β = 107°26′± 12′. The trial structure has been derived by the three-dimensional Patterson analysis and refined by the two-dimensional Fourier syntheses. The absolute configuration of the Cucurbitine molecule has been determined by the anomalous scattering effect of the chlorine atom with CuKx radiation. It is of the form: Some important features of the structure of both the molecule and the crystal are described.