One of the most challenging areas in plant pathology is the isolation of genes that control a plant''s interaction with pathogens. The difficulty in isolating these genes stems from the complex nature of most higher plants. We suggest that the cruciferous weed Arabidopsis thaliana may serve as a model organism in plant pathology since its biological and genetic properties and genomic structure are particularly amenable to the isolation of plant genes. We report here that Arabidopsis was a host for the phytopathogenic bacterium Xanthomonas campestris pv. campestris, the causal agent of black rot in cruciferous crops. Furthermore, two ecotypes of Arabidopsis showed differential responses to X. c. pv. campestris, suggesting that disease resistance genes are present in the Arabidopsis gene pool and may be isolated in a straightforward manner.