Pseudomonas syringae pv. tomato strain DC3000 (Pto DC3000), which causes bacterial speck disease of tomato, has been used as a model pathogen because of its pathogenicity on Arabidopsis thaliana. Here, we demonstrate a rapid and reliable flood-inoculation method based on young Arabidopsis seedlings grown on one-half strength MS medium. We also describe a method to evaluate the bacterial growth in Arabidopsis.