Crystals of gamma-chymotrypsin grown in aqueous solution were soaked in n-hexane, and the structures of both the soaked and the native crystals were determined to 2.2-A resolution. Seven hexane molecules and 130 water molecules were found in the hexane-soaked crystals. Two of the seven hexane molecules are found near the active site, and the rest are close to hydrophobic regions on or near the surface of the enzyme. In the hexane structure, water molecules that were not observed in the native structure form a clathrate around one of the hexane molecules. Only 97 water molecules were found in the native structure. The temperature factors for atoms in the hexane environment are lower than those in the aqueous environment. There are significant changes between the two structures in the side chains of both polar and neutral residues, particularly in the vicinity of the hexane molecules. These changes have perturbed the hydrogen-bonding patterns. The electron density for the peptide bound in the active site has been dramatically altered in hexane and appears to be tetrahedral at the carbon that is covalently bound to Ser 195. The crystalline enzyme retains its active conformation in the nonpolar medium and can catalyze both hydrolysis and synthesis reactions in hexane.