) has been widely used to control algae and pathogens in fish culture ponds. However, its toxic effects on fish depend not only on its concentration in water but also on water quality. The susceptibility of the rockfish, Sebastes schlegeli to copper was evaluated at three water-temper-atures (WT; 18, 23 and 28℃) for 4 days. After the exposure of two copper concentrations (100 and 200 μg/L), a hematological effect was exerted on rockfish, by causing changes in red blood cell count and hematocrit value at 28℃. Total protein levels of the fish showed a tendency of co-in-crease with glucose depend on the WT, after copper exposure. However, the plasma calcium and magnesium levels were significantly increased at 200 μg/L copper, regardless of the WT. Enzymes activities including ALT and LDH in serum were also significantly increased depend upon the copper treatment only. This indicates that inorganic components and enzymes activities were sensitive indexes to stress by toxicant such as copper. The cortisol levels were significantly elevated by both WT rising and copper treatment in serum of rock fish. In conclusion, these changes can be seen as an initial response to temperature stress and as a sustaining response to copper exposure. The present findings suggest that a simultaneous stress by temperature change and copper exposure could accelerate an alteration of hematological and plasma biological parameters in the rockfish. Key words: Sebastes schlegeli, Copper, Temperature, Hematology, Serum enzymes, Cortisol