Race-specific interaction between the Brassica plants and Xanthomonas compestris pv. campestris bacteria follows the "gene-for-gene" rule. Expression of the avirulence genes recognized by two dominant resistance genes of Brassica, Rxc1 in plants with the BB genome, and Rxc3 in the CC plants, was lost after bacterial mutation in planta. The mutation was distinguished by the elongation of CGCGC pentanucleotide repeat in the gene, which was designated as avrRxc1/3. This gene displayed strong structural similarity to the avrBs2 locus from the related species X. vesicatoria. Thus, it is the first description of the avrRxc1/3 avirulence gene conferring race-specific interaction between X. campestris pv. campestris and Brassica plants. Structural homologues of the avrBs2 are found in many Xanthomonas species, but in all cases except X. vesicatoria, their function remains unknown.