The flhD gene from Escherichia coli codes for one of the transcriptional activators of flagellar genes and is a putative global regulator in the cell. FlhD together with FlhC forms a complex that positively regulates the expression of flagellar genes. However, FlhD is able to change ifs DNA binding specificity, probably by forming another complex with some unknown factor or by binding to other targets by itself. FlhD is a good model to study the DNA binding specificity by complex formation. The FlhD three-dimensional structure will certainly improve our understanding of this protein, which is the first case of combinatory specificity in prokaryotes. The flhD gene was over-expressed, and the resulting protein was purified to homogeneity. FlhD crystals were obtained from 15-25% PEG 5000, 0.05-0.1 M lithium sulfate, 0.1 M Tris, pH 8.5, or 20-30% PEG 5000, 0.05-0.2 M sodium acetate, and 0.1 M Tris-HCl, pH 8.5, as precipitant solutions, and belong to the space group I4 with unit cell parameters of a = b = 90 Angstrom, c = 42.6 Angstrom and alpha = beta = gamma = 90 degrees and contain two molecules per asymmetric unit. A complete native data set has been collected to 2.3-Angstrom resolution.