A B S T R A C TThe antibacterial activity of chitosan solution against Agrobacterium tumifaciens was investigated in this study. The in vitro antibacterial effect of chitosan against A. tumifaciens was affected by chitosan concentrations, pH value, concentration of acetic acid used to dissolve chitosan and incubation time. Chitosan concentrations 2.5 mg mLG 1 and 5 mg mLG 1 exhibited strong antibacterial activity at the pH 6.6 and 5.6, respectively. The gall diameter and gall weight of tomato seedlings at dipping time 10 and 20 min was significantly reduced by chitosan concentrations 2.5 and 5 mg mLG 1 at pH 6.6 and 5.6, respectively. The viable bacterial counts after 8 h of incubation in chitosan concentration 5 mg mLG 1 at pH 5.6 was 0.0 log CFU mLG 1 .