The objective of this study was to determine the effect of sublethal copper concentrations on certain antioxidant enzymes and lipid peroxidation products in the postlarvae (PL) of Penaeus indicus when subjected to short- and long-term exposure in the laboratory. The PL of P. indicus were exposed to 0.1641 ppm (sublethal) copper for a period of 30 days along with a parallel control. Sampling was carried out at six different time intervals, i.e., 24, 48, and 96 hrs (shortterm), and 10, 20, and 30 days (long-term). Variations in the activity of the antioxidant enzymes, namely, catalase (CAT) and superoxide dismutase (SOD), as well as lipid peroxidation products (LPP) were measured as biomarkers of metal toxicity. Our results showed a significant (P < 0.05) increase in LPP (indicating oxidative stress) and CAT activity (indicating an adaptive response of the PL for protection against oxidative stress) in the exposed PL for all periods of exposure. However, SOD activity significantly (P < 0.05) decreased on 20 and 30 days exposure, indicating susceptibility of the PL to oxidative stress upon long-term exposure. Therefore, CAT can serve as a better biomarker of oxidative stress than SOD to long-term copper toxicity. Our results indicate that copper contamination causes oxidative stress even at sublethal doses in Penaeus indicus PL, which can thus be used as a potential biomarker of copper toxicity for long-term monitoring of coastal marine ecosystems.