Styela clava were exposed to sublethal concentrations of methamidophos (5.0, 10.0 and 15.0 μg/L) for 96 h, its biochemical stress response was studied taking into account carboxylesterase (CbE), catalase (CAT), SOD and glutathione S-transferase (GST) activities, as well as reduced glutathione (GSH) levels in tissues. In all exposed groups CbE was significantly inhibited. GSH level was reduced after exposure, and CAT activity decreased continuously. 24 h exposure to methamidophos did not affect GST activity. The results show that the early inhibitory effect in CAT activity and GSH content produced by methamidophos may be associated with a high degree of oxidative stress. CbE inhibition is a good biomaker for methamidophos exposure. This study revealed that S. clava developed adaptive response to neutralize the oxidative stress following pesticide exposure and that SOD and GPx can be used as biomarker in determining methamidophos toxicity due to its early response at even low concentration levels.