Wastewater from municipal, agricultural and industrial sources is a pervasive contaminant of aquatic environments worldwide. Most studies that have investigated the negative impacts of wastewater on organisms have taken place in a laboratory. Here, we tested whether fish behaviour is altered by exposure to environmentally relevant concentrations of wastewater effluent in the field. We caged bluegill sunfish (Lepomis macrochirus) for 28 days at two sites downstream (adjacent to and 870 m) from a wastewater treatment plant and at a reference site without wastewater inputs. We found that exposed fish had a dampened response to simulated predation compared to unexposed fish, suggesting that fish may be at greater risk of predation after exposure to wastewater effluent. Fish held at the different sites did not differ in activity and exploration. Our results suggest that predator avoidance may be impaired in fish exposed to wastewater effluent, which could have detrimental implications for aquatic communities.