The crystal and molecular structure of the phthalide isoquinoline alkaloid bicuculline, C20H17NO6, has been determined by X-ray diffraction methods. The compound crystallizes in the orthorhombic space groupP212121 (No. 19) witha = 14·49,b = 10·90 andc = 10·43 A. The atomic parameters were refined by least squares to give a conventionalR-factor of 0·0437 for 1557 symmetry independent reflections.