A molecular genetic approach was used to study the interaction between Arabidopsis and an avirulence gene from Pseudomonas syringae pv. pisi. P. syringae pv. pisi strain 151 induces a hypersensitive response (HR) when inoculated on the Arabidopsis accession Po-1. A genomic cosmid library was constructed from DNA from P. syringae pv. pisi strain 151 and a cosmid was identified that causes the normally virulent P. syringae pv. tomato strain DC3000 to induce an HR on Po-1. The cosmid was subcloned and a 1.2-kb DNA fragment conferring avirulence activity was sequenced. The single significant open reading frame within the 1.2-kb fragment was designated avrRps4. An avrRps4-specific probe hybridizes to DNA from all P. syringae pv. pisi strains tested. P. syringae strains carrying avrRps4 induces an HR on specific accessions of both Arabidopsis and soybean. Arabidopsis accession Ws-0 is resistant to DC3000(avrRps4), whereas accession RLD is susceptible. Resistance segregates as a single dominant locus in a genetic cross between Ws-0 and RLD. This disease resistance locus, RPS4, was mapped to chromosome 5 between the molecular markers sAT2105 and KG-8.