The toxicity of copper and zinc sulphates to immature Atlantic salmon (Salmo salarL.) was tested in soft water.The relation between concentration of metal and survival-time could be fitted by a straight line when logarithms were used. A sharp break in this relation marked theincipient lethal level, where survival became indefinitely long. Incipient lethal levels were 48 μg/l of copper and 600 μg/l of zinc.In solutions containing both copper and zinc, fish died twice as fast as would occur if the 2 metals were simply additive in their lethal action. Resistance-times in zinc solutions were increased at pH 7.9–9.3, and results fitted the hypothesis that dissolved zinc was toxic, but not suspended zinc. Survival in a given concentration of zinc was 4 times as long at 5° as at 15 °C, and the incipient lethal level was at least 1.5 times higher, with fish acclimated to each temperature.