In order to crystallographically study the hydration of the major groove (convex surface) of Z-DNA, the oligonucleotide d(CGCGCG) has been synthesized. Single crystals were grown by vapor diffusion using the hanging-drop and sitting-drop methods for X-ray studies and by batch crystallization and evaporation within silicon tubes for neutron studies. Hexagonal crystals were obtained without the use of duplex-stabilizing polyamines and at an acid pH. X-ray data collected at room temperature (1.5 angstroms resolution; unit-cell parameters a = 17.90, b = 30.59, c = 44.61 angstroms) and at 100 K (1 angstroms resolution; a = 17.99, b = 30.98, c = 44.07 angstroms) and neutron data collected at room temperature (1.6 angstroms resolution; a = 18.00, b = 31.16, c = 44.88 angstroms) indicate that the DNA is in the Z-form packing in space group P2(1)2(1)2(1).