The structure of bromomexicanin-E (C₁₄H₁₅O₃Br) has been determined by X-ray diffraction study. The crystals belong to space group P2₁2₁2₁ with four molecules in the unit cell. The lattice parameters are a= 6·105, b= 12·171, and c= 17·961 Å. The position of the bromine atom was found from a three-dimension Patterson map. The first Fourier was based on bromine phases and the rest of the structure emerged gradually from three more Fouriers. Anisotropic least-squares refinement has reduced R to 6·2 percent. The structure shows both five-membered rings are cis-fused to the seven-membered rings, the two fusions being cis-syn-cis. The seven-membered ring is bent and in a boat form. The average C–C bond distance is 1·54 Å, the average CC distance is 1·33 Å, and the average CO bond distance is 1·22 Å. The thermal vibrations show considerable anisotropy.