The X-ray structure of bovine beta-lactoglobulin with the ligand 12-bromododecanoic acid as a model for fatty acids has been determined at a resolution of 2.23 A in the trigonal lattice Z form. The ligand binds inside the calyx, resolving a long-standing controversy as to where fatty-acid like ligands bind. The carboxylate head group lies at the surface of the molecule, and the lid to the calyx is open at the pH of crystallization (pH 7.3), consistent with the conformation observed in ligand-free bovine beta-lactoglobulin in lattice Z at pH 7.1 and pH 8.2.