In the environment, arsenic(As)exists in a number of chemical species, and arsenite(AsIII) and arsenate (AsV) dominate in freshwater systems. Toxicity of As species to aquatic organisms is complicated by their interaction with chemicals in water such as phosphate that can influence the bioavailability and uptake of AsV. In the present study, the toxicities of AsIII, AsV and dimethyl arsinic acid(DMA) to three freshwater organisms representing three phylogenetic groups: a phytoplankton (Chlorella sp. strain CE-35), a floating macrophyte (Lemna disperma) and acladoceran grazer (Ceriodaphnia cf. dubia), were determined using acute and growth inhibition bioassays (EC50) at a range of total phosphate (TP) concentrations in OECD medium. The EC50 values of AsIII, AsV and DMA were 27 + 10,1.15 + 0.04 and 19+3 mgL-1 for Chlorella sp. CE-35;0.57+0.16,2.3+0.2 and 56 + 15mgL-1 for L. disperma, and 1.58+0.05, 1.72+0.01 and 5.9+0.1mgL-1 for C. cf. dubia, respectively. The results showed that AsIII was more toxic than AsV to L. disperma; however,AsV was more toxic than AsIII to Chlorella sp. CE-35. The toxicities of AsIII and AsV to C. cf. dubia were statistically similar (p>0.05). DMA was less toxic than iAs species to L. disperma and C. cf. dubia, but more toxic than AsIII to Chlorella sp. CE-35. The toxicity of AsV to Chlorella sp. CE-35 and L. disperma decreased with increasing TP concentrations in the growth medium. Phosphate concentrations did not influence the toxicity of AsIII to eitherorganism. Chlorella sp. CE-35 showed the ability to reduce AsV to AsIII, indicating a substantial influence of phytoplankton on As biogeochemistry in freshwater aquatic systems.