The amphipods Hyalella azteca and Diporeia spp. were exposed to sediments dosed with dichlorodiphenyltrichloroethane (DDT), and the toxicity and toxicokinetics were determined. The toxicity was evaluated with the equilibrium partitioning (EqP) and critical body residue approaches. The DDT in the sediments degraded during the equilibration period prior to organism exposure. Thus, the toxicity using EqP pore-water toxic units (TUs) was evaluated for DDT and its degradation product, dichlorodiphenyld-ichloroethane (DDD), as the ratio of the predicted interstitial water concentration divided by the water-only LC50 values. The sum of TUs (ΣTU) was assumed to best represent the toxicity of the mixture. For H. azteca, the 10-d LC50 was 0.98 and 0.33 ΣTU for two experiments. For Diporeia spp., no toxicity was found in the first experiment with up to 3 ΣTU predicted in the interstitial water. However, in the second experiment, the 28-d LC50 was 0.67 ΣTU. These data suggest that the EqP approach approximately predicts the toxicity for the combination of DDT and DDD in sediment, provided a toxic unit approach is employed. The critical body residue approach also used TUs because DDT is biotransformed by H. azteca and because of the dual exposure to DDT and DDD. Because biotransformation was only determined in the second experiment, the critical body residue approach could only be evaluated for that case. The TUs were calculated as the ratio of the concentration in the live amphipods divided by the respective LR50 (residue concentration required to produce 50% mortality) values. The LR50 was 1.1 ΣTU for H. azteca for the 10-d exposure and 0.53 for Diporeia spp. after a 28-d exposure. Thus, this approach was also quite successful in predicting the toxicity. The accumulation and loss rates for H. azteca were much greater than for Diporeia spp. Thus, 10-d exposures represent steady-state conditions for H. azteca, while even at 28-d, the Diporeia spp. are not at steady state.