The aim of this study was to investigate the effect of temperature on oxidative stress in fish. The response of the European seabass, Dicentrarchus labrax, was investigated through a long-term experiment in captivity, where 0-group juveniles were exposed to temperatures that reflect the average summer temperature that they encounter in coastal (18 degrees C) and estuarine nurseries (24 degrees C), and also the temperature that they endure inside estuaries during heat waves (28 degrees C). The degradation product of lipid peroxidation, malondialdehyde (MDA). and catalase activity was assessed in white muscle throughout the long-term experiment (30 days). A similar pattern was observed for MDA and catalase activity. At 18 degrees C, there was an increase of MDA and catalase activity over time. At 24 degrees C, levels of MDA and catalase activity were lower than at 18 degrees C, and the increase in concentration over time was also lower. At 28 degrees C, MDA and catalase increased 2.7x and 6.9x, respectively, after 15 days, and then decreased to much lower values after 30 days. Oxidative stress response was not directly correlated to temperature. It was lowest at the optimal temperature (24 degrees C) and it increased outside this species upper and lower optimum thermal limits. It was concluded that, although these biomarkers have been used mostly as indicators of the effects of contamination in field studies, they are very sensitive to temperature. Thus, comparison of sites subjected to different thermal regimes should be done with caution. (C) 2012 Elsevier Ltd. All rights reserved.