Technical Abstract: A potential biocontrol agent of bacterial wilt, Bacillus subtilis SB1, isolated from tomato roots, showed a broad-spectrum of antimicrobial activity in in vitro experiments. It inhibited the growth of many plant pathogens, including Ralstonia solanacearum, Xanthomonas oryzae pv. oryzae, Fusarium oxysporum, Botrytis cinerea, Phytophthora capsici, and tomato mosaic virus. In vitro tests also revealed anti-fungal and anti-TMV activities of the cell-free culture filtrate. Greenhouse experiments, focused on tomato bacterial wilt caused by Ralstonia solanacearum, revealed that the inhibition of the disease in sterilized and non-sterilized soil by SB1 suspension was highly significant compared to controls. Inhibition was observed when the SB1 suspension was applied before or simultaneously with the pathogen, while less protection occurred when the SB1 suspension was applied after pathogen inoculation. These results indicated that the biocontrol effects of SB1 were more likely to be a preventive effect rather than a therapeutic effect on the disease. While Bacillus subtilis is rhizosphere competent, our results indicated that the cell-free culture filtrate was more effective as a biocontrol substance than the washed cell suspension. Antibiosis of the cell-free culture filtrate from SB1 may play a major role in the suppression of tomato bacterial wilt. Investigations into the possible mechanisms of the culture filtrate on the pathogen Ralstonia solanacearum revealed that the cell morphology of R. solanacearum was greatly altered by the culture filtrate, and the expression of two extra-cellular and two intra-cellular proteins of R. solanacearum were also inhibited.