There are three thioredoxin isoforms in the yeast Saccharomyces cerevisiae: two cytosolic/nuclear thioredoxins, Trx1 and Trx2, and one mitochondrial thioredoxin, Trx3. In the present work, S. cerevisiae Trx3 overexpressed in Escherichia coli was purified and crystallized. The Trx3 crystals were obtained by the hanging-drop vapour-diffusion method. A data set diffracting to 2.0 A resolution was collected from a single crystal. The crystal belongs to space group P3(1), with unit-cell parameters a = b = 49.57, c = 94.55 A, alpha = beta = 90, gamma = 120 degrees. The asymmetric unit is assumed to contain two subunits of Trx3, with a V(M) value of 2.62 A(3) Da(-1) and a solvent content of 53%.