Red Kidney bean ( Phaseolus vulgaris L.) plants inoculated with Pseudomonas phaseolicola (isolate HB-36) showed water-soaked infection centers on the inoculated leaf surfaces about 48 h after inoculation. In contrast, resistant bean plants (GN Nebraska 27) inoculated with the same isolate responded by developing a hypersensitive reaction (HR) after roughly the same period after inoculation. High concentrations of phenolic compounds were detected in the ethanolic extracts of leaf tissues showing HR. Among these were identified the isoflavonoid phytoalexins phaseollin (pterocarpan), phaseollinisoflavan (isoflavan), coumestrol (coumestan) and kievitone (isoflavanone). These isoflavonoids were detected in substantial quantities 24 h after inoculation and their concentrations increased further up to 72 h, after which time they began to decrease. In susceptible Red Kidney leaves, isolate HB-36 induced much lower concentrations of these compounds at all stages of disease development. In repeated in vitro bioassays the above four compounds consistently restricted colony formation by P. phaseolicola isolates. However, the race 2 isolates, HB-36 and G-50, were found to be comparatively more tolerant to these compounds than the race 1 isolates, HB-33 and HB-20. Colony formation in race 1 isolates was completely inhibited by phaseollin and phaseollinisoflavan at 42 and 21 μg/ml; colony formation of race 2 isolates was retarded (90 to 98%) but not completely suppressed at 52·5 μg/ml of phaseollin and 52 μg/ml of phaseollinisoflavan. Coumestrol and kievitone at 21 and 33 μg/ml respectively were totally toxic to race 1 isolates but they were relatively less toxic to race 2 isolates even at higher test concentrations. All four phytoalexins also restricted the growth of P. glycinea isolates, R-2 and 724 (Leben). Isolate 724, however, exhibited a greater degree of tolerance than isolate R-2. P. fluorescens, a saprophyte, was relatively insensitive to the phytoalexins and showed only 26 to 30% retardation in colony formation at the maximum test concentrations used. These studies show that the virulent isolates of P. phaseolicola are not as sensitive to bean phytoalexins as the relatively less virulent isolates of P. phaseolicola.