The toxicity and toxicokinetics of radiolabeled DDT and its major degradation products, dichlorodiphenyldichloroethane (DDD) and dichlorodiphenyldichloroethylene (DDE), were determined for the amphipods Hyalella azteca and Diporeia spp. in water-only static renewal exposures. Comparison of the water and tissue concentrations associated with decreased survival revealed large differences in toxicity among the three compounds. In H. azteca, the ratio of the 10-d LR50 values (median lethal tissue residue) for DDT:DDD:DDE was 1:24:195. In Diporeia spp., the 28-d LR50 for DDT was higher than that for DDD by a factor of six, and DDE did not cause significant mortality even at concentrations approaching the solubility limit. Based on the toxicity data, the hazard from exposure to mixtures of DDT and its degradation products should be evaluated on a toxic-units basis and not as a simple summation of the individual concentrations, which ignores the toxicity of specific compounds. Differences in species sensitivity were also detected. The 10-d LR50 values were higher in Diporeia spp. than in H. azteca by a factor of 40 for DDT and eight for DDD. This difference can be only partly attributed to differences in lipid content between H. azteca (7% dry wt) and Diporeia spp. (24% dry wt). The uptake clearance and elimination rate constants were similar among the various compounds in both species. Uptake clearance was typically fourfold greater for H. azteca than for Diporeia spp., however, and the experimentally measured elimination rate was approximately 30-fold greater in H. azteca than in Diporeia spp. The larger rates of uptake and elimination were attributed to the higher exposure temperature, greater surface area-to-volume ratio, and lower lipid content for H. azteca compared with Diporeia spp. In addition, extensive biotransformation of DDT by H. azteca may have contributed to a more rapid compound elimination.