Acetohydroxy acid isomeroreductase (AHIR) is involved in the biosynthetic pathway of branched-chain amino acids in microorganisms and plants. AHIR from Pseudomonas aeruginosa has been overexpressed in Escherichia coli and crystallized at 297 K using potassium/sodium tartrate as a precipitant. X-ray diffraction data have been collected to 2.0 A resolution at 100 K using synchrotron radiation. The crystals belong to the cubic space group P2(1)3, with unit-cell parameters a = b = c = 184.38 A, alpha = beta = gamma = 90 degrees. Six monomers are present in the asymmetric unit, giving a V(M) of 2.34 A(3) Da(-1) and a solvent content of 47.4%.