General biochemical biomarkers are widely used in current ecotoxicology and may function as early warning signals. We have, however, poor knowledge on how ecologically similar species differ in their biomarker responsiveness and how predation risk may affect these biomarkers, potentially in an interactive way with pesticides. We evaluated this by exposing four corixid water bug species to combinations of endosulfan and predation risk and quantifying the activity of four general enzymatic biomarkers: acetylcholinesterase (AChE), phenoloxidase (PO), catalase (CAT) and superoxidedismutase (SOD). AChE activity was inhibited at an endosulfan concentration of 2 mu g l(-1) and this did not differ significantly among species. Predation risk inhibited AChE activity with the same magnitude as endosulfan in one species, S. striata. Reduction in the investment of immune function following pesticide exposure, as measured by the activity of PO, was only observed in C. coleoptrata at 8 mu g l(-1) while we observed an increase of PO levels in S. striata. Overall, PO was suppressed under predation risk at 8 mu g l(-1) endosulfan. For SOD we observed a pesticide-induced increase across all species under predation risk, while for CAT the pesticide-induced increase was only present without predation risk. These results indicate that even within this group of ecologically similar and closely related species opposing biomarker responses may exist, as observed for PO. Effects of predation risk on all four enzymes, at a similar magnitude as the pesticide effects, further question their usefulness as general biomarkers.