Pseudomonas syringae, P. phaseolicola, and P. glycinea produced and were sensitive to bacteriocins. Both solid and liquid media were used in the detection procedures. The bacteriocins were classified into 11 groups on the basis of activity spectra against strains of the same species, related and unrelated species, and unrelated genera. Heat, chloroform, trypsin, and pronase sensitivity were useful in subdividing some of the groups. All P. syringae strains produced bacteriocins, as did 55% of the P. glycinea strains and 8% of the P. phaseolicola strains.Bacteriocins from P. syringae had the broadest spectrum of activity, those from P. glycinea were more restricted in activity, and those from P. phaseolicola had the narrowest range of activity.Bacteriocin production was affected by agar depth, age of plates, age of culture, colony type, amount of inoculum, proximity to producers of bacteriocin inhibitors, and temperature. In many cases, detection of bacteriocin production depended on the choice of these conditions.