Rubber oxygenase A (RoxA) from Xanthomonas sp. strain 35Y is an extracellular dioxygenase that is capable of cleaving the double bonds of poly(cis-1,4-isoprene) into short-chain isoprene units with 12-oxo-4,8-dimethyl-trideca-4,8-diene-1-al (ODTD) as the major cleavage product. Crystals of the dihaem c-type cytochrome RoxA were grown by sitting-drop vapour diffusion using polyethylene glycol as a precipitant. RoxA crystallized in space group P2(1), with unit-cell parameters a = 72.4, b = 97.1, c = 101.1 A, beta = 98.39 degrees, resulting in two monomers per asymmetric unit. Diffraction data were collected to a limiting resolution of 1.8 A. Despite a protein weight of 74.1 kDa and only two iron sites per monomer, phasing was successfully carried out by multiple-wavelength anomalous dispersion.