Aims: Previously, we selected a bacterial strain (GSE09) antagonistic to Phytophthora capsici on pepper, which produced a volatile compound (2,4-di-tert-butylphenol), inhibiting the pathogen. In this study, we identified strain GSE09 and characterized some of the biological traits of this strain in relation to its antagonistic properties against P.similar to capsici. In addition, we examined bacterial colonization on the root surface or in rhizosphere soil and the effect of various concentrations of the volatile compound and strain GSE09 on pathogen development and radicle infection as well as radicle growth. Methods and Results: Strain GSE09 was identified as Flavobacterium johnsoniae, which forms biofilms and produces indolic compounds and biosurfactant but not hydrogen cyanide (HCN) with little or low levels of antifungal activity and swimming and swarming activities. Fl.similar to johnsoniae GSE09 effectively colonized on pepper root, rhizosphere, and bulk (pot) soil, which reduced the pathogen colonization in the roots and disease severity in the plants. Various concentrations of 2,4-di-tert-butylphenol or strain GSE09 inhibited pathogen development (mycelial growth, sporulation, and zoospore germination) in I-plate (a plastic plate containing a center partition). In addition, germinated seeds treated with the compound (1100 mu g ml-1) or the strain (1021010 cells ml-1) significantly reduced radicle infection by P.similar to capsici without radicle growth inhibition. Conclusions: These results indicate that colonization of pepper root and rhizosphere by the Fl.similar to johnsoniae strain GSE09, which can form biofilms and produce indolic compounds, biosurfactant, and 2,4-di-tert-butylphenol, might provide effective biocontrol activity against P.similar to capsici. Significance and Impact of the Study: To our knowledge, this is the first study demonstrating that the Fl.similar to johnsoniae strain GSE09, as a potential biocontrol agent, can effectively protect pepper plants against P.similar to capsici infection by colonizing the roots.