Xanthomonas campestris pv. campestris is a Gram-negative yellow-pigmented pathogenic bacterium that causes black rot, one of the major worldwide diseases of cruciferous crops. Its genome contains approximately 4500 genes, roughly one third of which have no known structure and/or function. However, some of these unknown genes are highly conserved among several different bacterial genuses. XC229 is one such protein containing 134 amino acids. It was overexpressed in Escherichia coli, purified and crystallized using the hanging-drop vapour-diffusion method. The crystal diffracted to a resolution of at least 1.80 A. It is cubic and belongs to space group I2(x)3, with unit-cell parameters a = b = c = 106.8 A. It contains one or two molecules per asymmetric unit.