The avirulence gene avrPph3 from Pseudomonas syringae pv. phaseolicola was tested for its ability to convert virulent P. syringae pv. tomato strain DC3000 to avirulence on Arabidopsis. In F2 plants from a cross between resistant and susceptible ecotypes, the ratio of resistant to susceptible plants was approximately 3:1, indicating that resistance to DC3000(avrPph3) is determined by a single dominant locus, which we have designated RPS5. RPS5 was mapped to chromosome 1, between restriction fragment length polymorphism markers m241 and g3786.