Based on a preliminary structural model of cyclodextrin glycosyltransferase from Bacillus circulans (EC 2.4.1.19), Ser428 and Ser475 of the enzyme were mutated to cysteines in order to produce suitable heavy atom derivatives. Mutant Ser475----Cys could not be expressed as protein. Mutant Ser428----Cys was expressed in Escherichia coli and purified. It crystallized isomorphously and gave rise to a mercury derivative that improved the electron density map. The structural results show that the new mercury-binding site is in a pocket at the protein surface.