Received 29 November 2007 Accepted 25 January 2008 AaManA, a -1,4-mannanase from the thermoacidophile Alicyclobacillus acidocaldarius Tc-12-31, was expressed in Escherichia coli and purified in a form suitable for X-ray crystallographic analysis. Crystals were obtained using the hanging-drop vapour-diffusion method at 291 K using ammonium dihydrogen phosphate as a precipitant. Data were collected from native mannanase and from a selenomethionyl derivative to 1.90 and 1.99 Å, respectively, at 100 K. The native crystal belongs to the orthorhombic space group P212121, with unit-cell parameters a = 44.34, b = 75.55, c = 88.02 Å. The derivative crystal belonged to the same space group as native AaManA, with unit-cell parameters a = 44.55, b = 75.70, c = 92.66 Å.