The development of resistance to toxicants has been reported for a wide range of organisms. Two questions were addressed in the present study: Are genetically determined resistance responses at lethal levels (min to h) of copper associated with responses at extreme lethal (h to d) and sublethal levels and does genetically determined resistance to lethal levels of copper confer resistance to lethal levels of other chemicals? Twelve cloned lineages of Daphnia longispina, differing in their resistance to copper, were exposed to sublethal concentrations of copper and to lethal concentrations of copper, zinc, cadmium, hydrogen ions, and a pyrethroid insecticide (deltamethrin). Three kinds of toxicity assays were carried out: Survival time (death occurs in min to h; only for metals and hydrogen ions), cumulative mortality (death occurs in h to d; for all tested chemicals), and feeding depression assays (only for copper). A correlation between different levels of toxicity was observed only for extreme and moderate lethal responses to copper, and no correlation was found between lethal and sublethal levels of copper. Multiple resistance to lethal levels of toxicants was observed only for the pair copper/zinc.