Wastewater treatment works (WWTWs) are known to provide profitable foraging areas for insectivorous bats in Europe and the New World because of their association with high abundance of pollution-tolerant midges (Diptera). However, bats that feed on these insects may also accumulate metal pollutants such as cadmium and copper in their tissues, with acute or chronic effects on their health. Using a time expansion bat detector, the activity (number of passes and feeding buzzes) of Neoromicia nana (family Vespertilionidae) was quantified at three WWTW points - upstream, the point of wastewater effluent discharge, and downstream along three urban rivers in South Africa. In addition, metal concentrations in the kidney, liver and muscle tissue of N. nana caught at the sites were quantified. The diversity of aerial insects, sampled over the same period as the bat surveys, was measured using a black light trap and sweep-netting. Relative abundance and feeding activity of N. nana were higher at wastewater-polluted sites than at upstream sites. The most abundant insect order at wastewater-polluted sites and in the diet of resident bats was Diptera. Essential metals (copper, zinc and iron) were detected in all N. nana tissue samples, but the toxic metals cadmium, chromium and nickel were mostly present in tissue of bats at wastewater-polluted sites. Thus, although WWTWs provide an optimal food resource to bats in the short-term, it may pose serious long-term health risks for these top predators.