X-Ray crystal-structure analysis of tetra-O-methylergoflavin di-p-iodobenzoate has defined the constitution and absolute stereochemistry of ergoflavin, an ergot pigment, to be as shown in (II). The crystals are orthorhombic, space group P212121, with four molecules of C48H40I2O16 in a unit cell of dimensions a= 13·23, b= 38·70, c= 9·37 Å. The crystal structure was elucidated by the heavy-atom method and the atomic co-ordinates were refined by Fourier and least-squares calculations. The final value of the discrepancy R over 3188 independent reflexions is 14·3%.
