Abstract: In this work, an endophytic bacterial strain ST6 with chlorpyrifos-degrading ability was isolated from chlorpyrifos-contaminated rice (Oryza sativa L.) and identified as Aeromonas hydrophila by 16S rRNA analysis. The strain ST6-gfp was successfully colonized in rice plants after being labeled with the green fluorescent protein gene (gfp), and the colony number of ST6-gfp in rice roots was 7.0-7.2 log CFU/g, and that in stems and leaves was 4.9-5.1 log CFU/g by plate recovery tests. The effect and the mechanisms of the endophytic bacteria colonization on the degradation of chlorpyrifos was investigated with pot experiments. The results show that the interaction between the plant and the endophytic bacteria promotes the degradation of chlorpyrifos and the growth of rice, and the content of chlorpyrifos in ST6-gfp inoculated rice is 58.3% lower than that in the uninoculated rice. The plant height, fresh weight and chlorophyll content of the inoculated rice increase by 38.9%, 69.5%, and 41.5%, respectively, compared with the uninoculated control. Under chlorpyrifos stress, ST6-gfp colonization enhancs the activity of oxidoreductase and detoxification enzymes in rice, in which the activities of superoxide dismutase (SOD), peroxidase (POD), ascorbate peroxidase (APX), glutathione reductase (GR), catalase (CAT), and phenylalanine aminolase (PAL) enzymes increased by 1.1, 1.3, 1.2, 1.8, 1.2 and 1.7 fold, respectively; the activities of cytochrome P450 (CYP450) and glutathione S-transferase (GST) were up-regulated by 1.5-1.7 fold. In addition, the MDA content of inoculated rice was reduced by 46.9% compared with that of uninoculated rice. This study suggests that ST6 colonization accelerates the degradation of chlorpyrifos in rice by increasing the activities of antioxidant and detoxification enzymes, thus reducing the stress damage caused by chlorpyrifos and improving the tolerance of rice to chlorpyrifos.