MicA is a response regulator from Streptococcus pneumoniae thought to be involved in redox-energy sensing under oxygen-limiting environments. The purified protein was crystallized using the sitting-drop vapour-diffusion technique. X-ray diffraction data were collected using synchrotron radiation to a resolution of 1.91 A. The crystals belong to the monoclinic space group C222(1), with unit-cell parameters a = 78.69, b = 92.57, c = 37.16 A, alpha = beta = gamma = 90.0 degrees. The Matthews coefficient indicates that MicA crystallizes with one molecule in the asymmetric unit.