IS200 transposases, originally identified in Salmonella typhimurium LT2, are present in many bacteria and archaea and are distinct from other groups of transposases. To facilitate further structural comparisons among IS200-like transposases, structural analysis has been initiated of a putative transposase from Thermoplasma acidophilum encoded by the Ta0474 gene. Its 137-residue polypeptide shows high levels of sequence similarity to other members of the IS200 transposase family. The protein was overexpressed in intact form in Escherichia coli and crystallized at 297 K using a reservoir solution consisting of 100 mM Na HEPES pH 7.5 and 20%(v/v) ethanol. X-ray diffraction data were collected to 1.78 A. The crystals belong to the monoclinic space group P2(1), with unit-cell parameters a = 65.00, b = 34.07, c = 121.58 A, alpha = 90, beta = 100.20, gamma = 90 degrees. Four monomers, representing two copies of a dimeric molecule, are present in the asymmetric unit, giving a crystal volume per protein weight (V(M)) of 2.02 A(3) Da(-1) and a solvent content of 39.2%.