The nucleotide-binding domain of the Na,K-ATPase ion pump was expressed with a His tag in Escherichia coli and purified. The soluble 24 kDa derivative consists of 214 amino-acid residues and was crystallized in the presence of NiCl(2). The crystals belong to space group F23, with unit-cell parameters a = b = c = 147.5 A, and diffract to 3.1 A. Complete data sets could be collected from native and thimerosal-treated crystals frozen in 50% sucrose. Five mercury positions were found and initial SIR phases calculated.