Two crystal forms of cytochrome c peroxidase from Pseudomonas nautica were obtained, one at pH 4.0 using sodium citrate as precipitant and another at pH 5.3 using ammonium phosphate and sodium citrate as precipitants. The two forms belong to different space groups P3(1)21 (pH 4.0) and P6(4)22 (pH 5.3), with unit-cell parameters a = b = 114.5, c = 90.7 A and a = b = 151.0, c = 155.9 A, respectively. Several complete data sets were collected using synchrotron radiation at ESRF and Cu K(alpha) X-ray radiation from a rotating-anode generator. These results will contribute to clarifying the haem transitions occurring during peroxidatic reaction and the required electron-transfer processes and to elucidating the catalytic mechanism of the enzyme and the role of calcium in the activation process.