The three-dimensional structure of a ligand-free closed form of the glucose/galactose binding protein from Salmonella typhimurium has been determined at a resolution of 1.9 A. The crystallographic R-factor for the refined structure is 17.9%. The model contains all the atoms of the 309 residues of the protein sequence, a calcium ion, and 174 water molecules. The root mean square (r.m.s.) deviations for the whole molecule are: 0.010 A for bond lengths and 2.44 degrees for bond angles, indicating a good stereochemistry for the model. This structure shows that the protein is able to close in the absence of ligand, adopting a conformation similar to the liganded form but slightly more open. Water molecules satisfy the hydrogen bonding ability of the hydrophilic side chains of the binding site in a manner which is reminiscent of the sugars' hydrogen-bonding patterns. Since packing forces are weak, the crystallization event is unlikely to trigger a change from an open to a closed conformation. Instead, the latter must be one of the species in equilibrium in solution which is selected by packing in the crystal lattice.