The cruciferous weed Arabidopsis thaliana and the causal agent of black rot disease of Crucifers Xanthomonas campestris pv. campestris ( Xcc ) are both model organisms in plant pathology. Their interaction has been studied successfully in the past, but these investigations suffered from high variability. In the present study, we describe an improved Arabidopsis- Xcc pathosystem that is based on a wound inoculation procedure. We show that after wound inoculation, Xcc colonizes the vascular system of Arabidopsis leaves and causes typical black rot symptoms in a compatible interaction, while in an incompatible interaction bacterial multiplication is inhibited. The highly synchronous and reproducible symptom expression allowed the development of a disease scoring scheme that enabled us to analyse the effects of mutations in individual genes on plant resistance or on bacterial virulence in a simple and precise manner. This optimized Arabidopsis- Xcc pathosystem will be a robust tool for further genetic and post-genomic investigation of fundamental questions in plant pathology.