Root discs of 4-year-old ginseng, Panax ginseng C. A. Meyer, were inoculated with the higher (108 colonyforming units (CFU)/ml) and lower (106 or 105 CFU/ml) initial inoculum levels of a plant-growth promoting rhizobacterium (PGPR), Paenibacillus polymyxa GBR- 1 to examine rot symptom development and bacterial population changes on the root discs. At the higher inoculum level, brown rot symptoms developed and expanded on the whole root discs in which the bacterial population increased continuously up to 4 days after inoculation. In light and electron microscopy, ginseng root cells on the inoculation sites were extensively decayed, which were characterized by dissolved cell walls and destructed cytoplasmic contents. However, no rot symptoms were developed and the bacterial population increased only during the initial two days of inoculation at the lower inoculum level (106 CFU/ml) of P. polymyxa GBR-1. At the lower inoculum level (105 CFU/ml), boundary layers with parallel periclinal cell divisions, structurally similar to wound periderm, were formed internal to the inoculation sites, beneath which the cells were intact containing numerous normal-looking starch granules and no disorganized cell organelles, suggesting that these structural features may be related to the suppression of symptom development, a histological defense mechanism.