Bacteriocins are narrow-spectrum antibacterial protein toxins produced by bacteria to inhibit the growth of closely related species. A fitness benefit of bacteriocin production has been shown for the producing strain in structured environments (e.g. agar plate surface). To explore this observation in planta, we performed a series of co-infiltration experiments of bacteriocin-producer Pseudomonas syringae pv. syringae (Psy), or bacteriocin-deficient mutant of Psy (Psy D_bac), against bacteriocin-sensitive P. syringae pv. phaseolicola (Pph). Populations of each strain were measured over an 8-day period. In 1:1 co-infiltrations at day 4, bacteriocin production showed an increase in killing of sensitive cells with a 100- and 10-fold reduction of Pph by Psy and Psy D_bac, respectively. By day 6, there was a 100-fold decrease of Pph by both strains removing the initial benefit observed at day 4. When starting at a low cell density (1:9), Psy achieved a significantly greater population at day 6 compared to Psy D_bac, showing a bacteriocin production benefit when in the minority. Moreover, Psy reduced Pph to a larger extent than Psy D_bac for all sample days, indicating a combination of bacteriocin-mediated antagonism and resource competition between co-colonizing bacteria in planta. Our data demonstrates that under specific conditions bacteriocin production can promote Psy fitness and reduce the growth of a competing strain.