This study examined the temporal component of pentachlorobenzene lethal body residues among three freshwater invertebrates. Also, using previous fluoranthene data allowed a more detailed examination of the role of biotransformation in lethal body residues and comparisons of lethal residues across chemical classes. Time-dependent toxicity of fluoranthene and pentachlorobenzene were compared among Hyalella azteca, Chironomus tentans, and Diporeia spp. Lethal body residues required for 50% mortality (LR50) were not constant and decreased with exposure time for all species. Fluoranthene was most toxic to C. tentans with LR50 values of 0.38 μmol·g−1 at 2 d to 0.15 μmol·g−1 at 10 d and least toxic to Diporeia spp. with values of 9.97 μmol·g−1 at 10 d to 3.67 μmol·g−1 at 28 d. The LR50 values for H. azteca were intermediate and ranged from 2.25 μmol·g−1 at 5 d to 0.56 μmol·g−1 at 28 d. Pentachlorobenzene LR50 values were less variable among species and ranged from 1.20 μmol·g−1 at 4 d to 0.81 μmol·g−1 at 10 d for C. tentans, 5.0 μmol·g−1 at 20 d and 2.75 μmol·g−1 at 28 d for Diporeia spp., and 1.51 μmol·g−1 at 4 d and 0.71 μmol·g−1 at 28 d for H. azteca. When LR50 values for fluoranthene and pentachlorobenzene were compared at steady state, the lethal residues for the amphipod species were within the range expected for nonpolar narcotic chemicals (anesthetics); however, C. tentans was more sensitive to fluoranthene than pentachlorobenzene, confirming our previous hypothesis that biotransformation of fluoranthene likely produces a metabolite(s) acting by some specific mechanism of action. The information collected from this study allows a greater understanding of residue-response relationships, specifically relative species sensitivities.