In this study, the joint acute toxicity of tributyl phosphate (TBP) and triphenyl phosphate (TPP) was investigated using Daphnia magna as the test organism. The median lethal concentrations for TBP and TPP at 24 and 48 h were 5.48 and 0.51 mg/L, 1.17 and 0.089 mg/L, respectively. When mixed at either equal concentration or various toxic unit ratios, TBP and TPP mixtures displayed an additive toxicity after both 24- and 48-h exposure. This work suggests that the joint acute toxicity of TBP and TPP towards D. magna can be predicted by calculating the toxic units of mixtures.