Crystalline gamma-chymotrypsin soaked in aqueous and nonaqueous solutions containing the inhibitor N-acetyl-D-tryptophan has been studied by X-ray crystallographic methods at a resolution of 2.2 Angstrom. This inhibitor has been used to induce "enzyme memory" in chymotrypsin in nonaqueous solvents (Stahl, M. et al. J. Am. Chem.Soc. 1991, 113, 9366-9368). The two soak solutions consisted of (a) the inhibitor, water, and 2-propanol and (b) the inhibitor, hexane, and Zpropanol. In the nonaqueous environment, the indole ring of the inhibitor binds in the specificity pocket of the enzyme for L-amino acids and not in a new site as had been suggested by others (Tawaki, S.; Klibanov, A. M. J. Am. Chem. Sec. 1992, 114, 1882-1884). No inhibitor binding was observed in the aqueous structure even though the concentration of inhibitor was identical to that in the nonaqueous experiment. Five hexane and three 2-propanol molecules were located in the nonaqueous structure. One of the three 2-propanol molecules occupies the P1’ binding site. In the aqueous structure one 2-propanol was observed. In both structures, the addition of Z-propanol resulted in a dramatic increase in the number of observed water molecules. The differential binding affinity of N-acetyl-D-tryptophan in water and in a nonaqueous environment suggests a molecular explanation for the phenomenon of enzymatic memory.