The effect of temperature and copper on the RNA/DNA and protein/DNA ratios of Petenia kraussii juveniles was evaluated. Juveniles were acclimated for 15 days at temperatures of 22°C and 30°C and later kept in aquaria containing copper concentrations of 0.2 and 1.2 mg/L for each temperature for 30 days. They were then transferred to water containing no metal to be detoxified. The bioassays were performed statically with water and metal renewal every 24 hours. RNA/DNA levels decreased in the organisms kept at 22°C, the decrease being more noticeable on the juveniles exposed to the higher copper concentration. The same response was evidenced for the protein/DNA ratio. Copper had no significant effect on the RNA/DNA and protein/DNA ratios of the specimens exposed to 1.2 mg/L concentrations at 30°C, whereas a slight increase of these parameters was detected on the specimens exposed to 0.2 mg/L during the first week. P. krassii juveniles, particularly those kept at 30°C, did not reach referent RNA/DNA and protein/DNA values after 30 days. These results demonstrate that temperature changes are determinant of copper toxicity in the juveniles of this species.