The cDNA encoding Peking duck Cu,Zn superoxide dismutase (dSOD) was cloned and sequenced. The recombinant enzyme was overexpressed in Escherichia coli, purified to homogeneity and crystallized using the sitting-drop vapour-diffusion technique. Trigonal crystals of dSOD were obtained at 278 K at low ionic strength and around neutral pH. These crystals belong to space group P3(2)21, with unit-cell parameters a = 124.4, c = 163.5 A, gamma = 120 degrees. The asymmetric unit contains four dimers (eight monomers of Cu,Zn dSOD) and has a 56% solvent content, with a V(M) of 2.8 A(3) Da(-1). On a Rigaku R-AXIS IIc image-plate area-detector system, the crystal diffracted to 2.9 A. Unusual supermolecular double-helix packing with 9(2)2 non-crystallographic symmetry in crystals has been observed in the initial structural analysis.