CooA, a homodimeric haem-containing protein, is responsible for transcriptional regulation in response to carbon monoxide (CO). It has a b-type haem as a CO sensor. Upon binding CO to the haem, CooA binds promoter DNA and activates expression of genes for CO metabolism. CooA from Carboxydothermus hydrogenoformans has been overexpressed in Escherichia coli, purified and crystallized by the vapour-diffusion method. The crystal belongs to space group P2(1), with unit-cell parameters a = 61.8, b = 94.7, c = 92.8 angstroms, beta = 104.8 degrees. The native and anomalous difference Patterson maps indicated that two CooA dimers are contained in the asymmetric unit and are related by a translational symmetry almost parallel to the c axis.