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To test the hypothesis that rhizobacteria naturally present in soils may interfere with the extent of root colonization and plant growth promotion by plant growth-promoting rhizobacteria (PGPR), we studied two lodgepole pine PGPR (Bacillus polymyxa strains L6 and Pw-2) when inoculated singly and when coinoculated with a non-PGPR competitor (Curtobacterium flaccumfaciens PF322). Bacillus polymyxa Pw-2 and Curtobacterium flaccumfaciens PF322 were consistently found as endophytes, while Bacillus polymyxa L6 was never found within the root interior. Strains Pw-2 and L6 differed in the rate and type of growth promotion. Strain Pw-2 increased root growth (branching and elongation) and shoot biomass accumulation 6 and 9 weeks, respectively, after inoculation, while strain L6 increased primary root elongation and root biomass accumulation after 12 weeks. Seedlings coinoculated with Pw-2 and PF322 …