The thioredoxin system of the archaeon Sulfolobus solfataricus involves a number of different proteins: two thioredoxin reductases (SsTrxRB2 and SsTrxRB3), two distinct thioredoxins (SsTrxA1 and SsTrxA2) and a disulfide oxidoreductase (SsPDO). Here, the crystallization and preliminary crystallo- graphic analyses of SsTrxA1 and SsTrxA2, two dimeric proteins endowed with extraordinary thermal stability, are reported. In addition to the functional thioredoxin domain, both SsTrxA1 and SsTrxA2 present an extra N-terminal fragment of approximately 30 residues. Although crystallization trials have been conducted on both forms of the proteins, crystals that were suitable for X-ray crystallographic analyses have only been obtained for their truncated variants. The crystals of SsTrxA2 belonged to space group P2, with unit-cell parameters a = 28.27, b = 27.88, c = 62.06 A ̊ , = 92.34, and diffracted to 1.83 A ̊ resolution, whereas the crystals of SsTrxA1 belonged to space group P21, with unit-cell parameters a = 51.76, b = 75.09, c = 55.35 A ̊ , = 112.64 , and diffracted to 1.90 A ̊ resolution. The structures of the two proteins have been solved by molecular replacement.