In order to investigate the effects of a symbiotic bacterium on arsenite (As(III)) accumulation and transformation of C. salina, one symbiotic bacterial strain was isolated from culture medium of C. salina. Antibiotics were added to C. salina to obtain the axenic microalga, and a series of As(III) treatments (0, 75, 150, 300, 750mg/L) were applied to the non-axenic and axenic C. salina. After 7 days, the absorption, adsorption and accumulation of As(III) by C. salina were determined. The As speciation in the culture and algal cells were also analyzed and the rates of As(III) oxidation and removal by non-axenic and axenic C. salina were calculated. At the same time, the symbiotic bacterium was treated with different concentrations of As(III) for 7days, and with 300mg/L As(III) for different time. The oxidation and removal rates of As(III) in the bacterial culture medium were then calculated. Results show that the symbiotic bacterium was identified as Agrobacterium tumefacienss WB-1. Compared with the axenic C. salina, the non-axenic C. salina grew faster with higher tolerance to As(III). Moreover, in the presence of bacterium, As(III) accumulation by C. salina from the culture solution varied from 103.10 to 448.12mg/kg, the oxidation and removal rates of As(III) were 78.93%~96.88% and 18.92%~55.21%, respectively. These values were significantly higher than those of the axenic C. salina, which varied from 11.68 to 91.39mg/kg, 14.46%~26.39% and 12.82%~29.15%, respectively. The As(III) oxidation and removal rates in the non-axenic C. salina culture were also significantly higher than those in the A. tumefaciens cuture (4.51%~30.61% and 1.86%~16.19%, respectively). In addition, a small amounts of As(III) and methylated As were detected in the cells of non-axenic C. salina, while no methylated As was present in the axenic C. salina cells. Symbiotic bacteria promote the enrichment and transformation of As(III) by Chlorella salina.