The periplasmic leucine-binding protein is the primary receptor for the leucine transport system in Escherichia coli. We report here the structure of an open ligand-free form solved by molecular replacement and refined at 1.5-A resolution. In addition, two closed ligand-bound structures of the same protein are presented, a phenylalanine-bound form at 1.8 A and a leucine-bound structure at a nominal resolution of 2.4 A. These structures show the basis of this protein's ligand specificity, as well as illustrating the conformational changes that are associated with ligand binding. Comparison with earlier structures provides further information about solution conformations, as well as the different specificity of the closely related leucine/isoleucine/valine-binding protein.