The Mycobacterium tuberculosis chaperonin 10 (Mtcpn10) has been crystallized by the sitting-drop vapour-diffusion method. The crystals belong to the monoclinic space group P21, with unit-cell parameters a = 76.5, b = 87.9, c = 124.4 A, beta = 106.8 degrees. X-ray diffraction data were collected to 2.8 A. The self-rotation function and the molecular-replacement solution show that the asymmetric unit contains a dimer of heptamers related by twofold non-crystallographic symmetry. The two heptamers interact through interleaving flexible loops in a similar fashion to M. leprae and Gp31 cpn10. In addition to its role in protein folding, Mtcpn10 has unique effects on the growth of host cells and is a major immunogen in tuberculosis infections. The structure determination will permit the analysis of the amino acids identified as important for the protein-folding and cell-signalling activity of Mtcpn10.