RNA polymerase holoenzyme from Thermus thermophilus, consisting of six protein subunits (alpha(2), beta, beta', omega and sigma(70)) and having a total molecular mass of about 450 kDa, was purified and crystallized by the hanging-drop vapour-diffusion technique under mild near-physiological conditions. The crystals diffract beyond 3 A resolution. Careful analysis of diffraction data revealed that the crystals belong to space group P3(2), with unit-cell parameters a = b = 236.35, c = 249.04 A, and have perfect twinning along the threefold axis. A complete data set at 3 A resolution was collected and an unambiguous molecular-replacement solution was found using the structure of T. aquaticus RNA polymerase core enzyme as a search model. The refinement of structure and model building of the sigma(70) subunit is now in progress.