Nucleoside diphosphate (NDP) kinase contributes to the maintenance of cellular pools of all nucleoside triphosphates (NTPs) by catalyzing the transfer of the gamma-phosphoryl group from an NTP donor to an NDP acceptor. NDP kinase from the extreme thermophilic bacterium Thermus thermophilus HB8 was overexpressed in Escherichia coli and crystallized at 297 K using polyethylene glycol 8000 as the precipitant by means of the hanging-drop vapour-diffusion procedure. The crystals belong to the hexagonal system, space group P6(3)22, with unit-cell parameters a = b = 124.0, c = 104.9 A, alpha = beta = 90, gamma = 120 degrees. Assuming the asymmetric unit to contain two subunits, the calculated V(M) value was 3.7 A(3) Da(-1). The crystals diffracted X-rays generated by the synchrotron at SPring-8 to at least 1.9 A resolution and were suitable for high-resolution X-ray crystal structure determination.