Abstract: The additive index of coeffects was used to study the toxicities of Cu-Zn, Cu-triazophos, Zn-triazophos and Cu-Zn-triazophos on Rana limnocharis Boie tadpole. The acute toxicities of Cu, Zn and triazophos were in the sequence of Cu > triazophos > Zn. The 24, 48 and 96 h median lethal concentrations of Cu were 0.98, 0.70 and 0.43 mg/L respectively; the 24, 48 and 96 h median lethal concentrations of Zn were 60.0, 51.0 and 46.5 mg/L; the 24, 48 and 96 h median lethal concentrations of triazophos were 5.63, 5.47 and 5.30 mg/L. The additive toxicity of Cu-Zn was shown to be synergism, while that of Cu-triazophos was antagonistic; that of Zn-triazophos and Cu-Zn-triazophos was antagonistic when the tadpole was exposed to mixtures for 24 h; but that was synergism for 48 and 96 h.