The aldohexose dehydrogenase from the thermoacidophilic archaeon Thermoplasma acidophilum (AldT) is a 28 kDa molecular-weight enzyme that catalyzes the oxidation of various aldohexoses, with a preference for NAD+ rather than NADP+ as a cofactor. The recombinant AldT was crystallized using the hanging-drop vapour-diffusion technique at 293 K under several acidic conditions with polyethylene glycol (PEG) and ammonium sulfate as precipitants. Optimization of the initial crystallizations conditions yielded single crystals in solution containing 0.1 M sodium acetate pH 4.6, 18%(w/v) PEG 4000, 0.2 M ammonium sulfate and 15%(v/v) glycerol. An X-ray diffraction data set was collected to a resolution of 2.8 A.