It must be pointed out that both concentration sequences present the same lethal toxicity and correspond to approximately 0; 1.2; 3; 6 and 15% of the 96h-LC (see Results). 50 Cooper and zinc were tested singly or in mixture. Copper–zinc mixtures (approximately 1:2.5) were obtained adding the equipotent concentrations of these metals, e.g. 17 ppb 21 21 Cu 141 ppb of Zn . All experimental media were prepared employing the same water used for postlarvae acclimation. Experiments were performed under the same conditions (temperature, salinity and photoperiod).