The single toxicities (24h-EC50) and mixture toxicities of 18 substituted biphenyls to Daphnia magna were tested. Quantitative structure-activity relationships (QSARs) were developed from the single toxicities. Octanol-water partition coefficient (IgK(ow)) model, theoretical linear solvation energy relationship (TLSER) model and quantum chemistry parameter model were built for these compounds. It was found that the quantum chemistry parameter model had a good predictive capability. The study of mixture toxicities of substituted biphenyls showed that the joint toxicity mechanism was concentration addition. Half effective concentrations of mixtures (EC50mix) were predicted according to concentration addition. The predicted and observed values coincided rather well.