Arsenic is an abundant metalloid and a dangerous pollutant when in solution under the arsenate or arsenite forms-As(V) and As(III), respectively. Since its biological effects are expected to depend on the oxidation state and on speciation, effect of pH on either As(V) or As(III) speciation and resulting toxicity was investigated using the Microtox bioassay based on change in light emission by the luminescent bacteria Vibrio fischeri. Within a 5.0-8.0 pH range, EC50 values for As(V) were found to decrease as pH became basic, reflecting an increase in toxicity; whereas in the case of As(III), EC50 values were almost unchanged within a 6.0-8.0 pH range and lowered only at pH 9.0. HAsO42- and H2AsO3-were found to be the most toxic species. A statistical approach based on testing the null hypothesis of additive toxicity revealed an antagonistic effect between the arsenate chemical species. At low concentrations, As(V) was regularly found to be more toxic than As(III), independent of the pH value. Conversely, at high concentrations, the toxicity of both As(III) and As(V) was found to chiefly depend on pH, as a consequence of the strong influence of this parameter on the chemical speciation.