A thermostable multicopper oxidase from Thermus thermophilus HB27 (Tth-MCO) was successfully crystallized using the sitting-drop and hanging-drop vapour-diffusion methods. Crystallization conditions and preliminary X-ray diffraction data to 1.5 Å resolution obtained using synchrotron radiation at 100 K are reported. The crystals belonged to space group C222(1), with unit-cell parameters a = 93.6, b = 110.3, c = 96.3 Å. A monomer in the asymmetric unit yielded a Matthews coefficient (V(M)) of 2.60 Å(3) Da(-1) and a solvent content of 53%. An inactive enzyme form, apo-Tth-MCO, was also crystallized and diffraction data were collected to 1.7 Å resolution. In addition, a second inactive form of the enzyme, Hg-Tth-MCO, was obtained by soaking apo-Tth-MCO crystals with mercury(II) chloride and data were collected to a resolution of 1.7 Å.