Disease resistance is believed to play a role in the dynamics of plant communities and plant populations. Here we study the competitive ability of two Arabidopsisthaliana genotypes, one exhibiting resistance and the other exhibiting susceptibility to an isolate of Peronospora parasitica. The A.thaliana genotypes were grown in competition experiments under controlled conditions at three densities in pure and mixed stands both in the presence and absence of P. parasitica. After seed set, the dry weight of the plants was determined and the amount of seeds produced was calculated from an established linear relationship. The two genotypes were found to be ecologically different; the susceptible genotype was the strongest competitor (significantly so in the absence of the disease), whereas the resistant genotype produced most seeds. The competitive ability of the disease resistant A.thaliana genotype increased significantly in the presence of a P. parasitica isolate when competing with the susceptible A.thaliana genotype, whereas the disease did not affect the competitive ability of the susceptible genotype significantly. Assuming that the population only consisted of the two genotypes and mainly was affected by the disease it was possible to predict the probabilities of four long-term ecological scenarios. Without the disease the most likely long-term ecological scenario was that the two A.thaliana genotypes would coexist, whereas, in the presence of the disease, the resistant genotype most likely would outcompete the susceptible genotype.