Abstract: Titanium dioxide nanoparticles (nTiO₂) have a higher capacity to adsorb contaminants, which in turn affects the biological accumulation and toxicity. To examine the toxicity and accumulation of arsenite (As(Ⅲ)) in Daphnia magna in the presence of nTiO₂ as transport carriers, the half inhibitory concentration (IC₅₀), and As and Ti accumulated levels were determined for different As concentrations (75 μg/L and IC₅₀) in absence and presence of nTiO₂ at 2 and 20 mg/L. Results showed that nTiO₂ can adsorb As(Ⅲ) to be balanced after 30 min. Specifically, the adsorption rate of As(Ⅲ) was 31.38% and 51.84%, respectively at 2 and 20 mg/L nTiO₂, and the amount of accumulated arsenic in D. magna was 2.9 and 3.8 times the amount accumulated in the control (without nTiO₂). This indicates that nTiO₂ could function as carrier and enhance the accumulation of As(Ⅲ) in D. magna. Meanwhile, the amount of accumulated arsenic in D. magna transported by nTiO₂ might decreased with increasing exposed As(Ⅲ) levels. The observation result is supported by the relationship between arsenic and titanium in D. magna for different As exposed levels. Furthermore, in spite of the increasing arsenic accumulation in D. magna with the nTiO₂ concentration, the toxicity of As(Ⅲ) did not increase. Specifically, the 24 h IC₅₀ of As(Ⅲ) increased from 0.93 mg/L in absence of nTiO₂ to 2.53 and 2.97 mg/L at 2 and 20 mg/L of nTiO₂, respectively, demonstrating that nTiO₂ decreased the toxicity of As(Ⅲ) on D. magna. The study concluded that nTiO₂ can enhance the accumulation of As(Ⅲ) in D. magna, but reduced its toxicity, which will be helpful of providing insights into ecological risks of combined pollution of nTiO₂ with other heavy metals.