The three-dimensional structure of d[CGTA'CG], where A' = [2-NH2-A], was determined to atomic (1.35 A) resolution by single isomorphous replacement. The d[CGTA'CG] hexamer crystallizes in space group P3221, and is not isomorphous with other DNA hexanucleotides. Despite completely different crystal packing, the essential characteristics of the Z-DNA conformation are maintained. The structure was determined by single isomorphous replacement using a triammine platinum fragment. Thus, this study also demonstrates, for the first time, the feasibility of the use of this reagent for the direct phasing of DNA crystal structures.