We have examined the genetics of nonhost resistance in Arabidopsis, using the bean pathogen Pseudomonas syringae pv. phaseolicola race 6 1448A to probe accessions for natural variation in basal defense. Symptoms rarely developed in leaves of Niedersenz (Nd), some yellowing and occasional necrosis developed in Columbia (Col), whereas tissue collapse was observed in Wassilewskija (Ws) after inoculation by infiltration. Analysis of F2 progeny and recombinant inbred lines (RIL) from a cross between Col and Nd revealed a pattern of continuous symptom increase, indicating the operation of quantitative determinants of resistance. By mapping quantitative trait loci (QTL), significant linkage was determined for resistance (low symptom score) to markers on chromosome 4. Segregation in the F2 cross from Nd × Ws indicated the operation of two dominant genes for resistance, one of which was FLS2 encoding the flagellin receptor. The requirement for FLS2 to confer resistance was confirmed by transgenic experiments, and we showed that the response to P. syringae pv. phaseolicola was affected by FLS2 gene dosage. Using RIL, the second locus was mapped as a QTL to a large interval on chromosome 1. Both FLS2 and the QTL on chromosome 1 were required for the highest level of resistance to bacterial colonization and symptom development in Nd.