This study aimed to investigate the compound effect of environmentally relevant 4-nonylphenol (NP) concentrations and natural stressors-namely fish predation and food availability-on Daphnia magna, which were exposed to four NP concentrations (0, 1, 5 and 10 mu g l(-1)) under optimum or low food concentrations (1.00 and 0.075 mg C l(-1), respectively) in water (un)conditioned by a fish predator (Alburnus alburnus). A(n) "environmentally relevant" and "no observable effect" concentration (NOEC) of NP (10 mu g l(-1)) resulted in a significant reduction (P < 0.01**) in daphnids' survival when it was encountered concurrently with conditions of low food availability and presence of fish predation. The significance of the results lies in the observation that not only environmentally relevant concentrations of NP but also NP concentrations reported to have no observable effect on daphnids may in reality have unexpected critical effects on D. magna survival under conditions more parallel to natural ecosystems. The deterioration of the life-history traits-namely, NP-induced delay in the age at first reproduction (P < 0.001***) and fish kairomone-induced reduction in the size at first reproduction (P < 0.001***)-of the D. magna individuals is also crucial, as such alterations could significantly influence future generations and result in ultimate adverse effects at the community level because large-bodied daphnids are key-stone species in freshwater ecosystems. The results of this study demonstrate the importance of taking into account environmentally realistic conditions while investigating the effects of NOEC levels of toxicants on non-target aquatic species.