Class II alpha-mannosidase cleaves off alpha-1,2-, alpha-1,3- and alpha-1,6-mannose residues. In this paper, the crystallization and preliminary X-ray analysis of cytosolic class II alpha-mannosidase from Thermotoga maritima (TM1851), a family 38 glycoside hydrolase, is described. The crystal of recombinant TM1851 belongs to the C-centred monoclinic space group C2, with unit-cell parameters a = 244.7, b = 87.4, c = 166.6 A, beta = 124.7 degrees. X-ray diffraction data were collected to a resolution of 2.9 A.