We assessed possible ecotoxicological implications of ozone application to secondary treated wastewater from a municipal wastewater treatment plant on Gammarus fossarum, an aquatic leaf shredding amphipod. Our 10-week study exposed G. fossarum populations to ozone-treated, non-ozone treated wastewater, or tap water in replicated outdoor flow-through stream microcosms. Feeding activity, an indicator for organic matter decomposition, of amphipod populations exposed to ozone treated wastewater was significantly higher compared to those exposed to non-ozone treated wastewater (repeated measure ANOVA, p = 0.0002, df = 44). Also the population size was at the end of the experiment with approximately 150% significantly (t-test, p = 0.0059, n = 4) increased in ozone treated wastewater compared to non-ozone treated wastewater. Additionally, chlorophyll-a concentration, an indicator for algal biomass, was significantly higher in ozone treated wastewater (repeated measure ANOVA, p = 0.0404, df = 65). Thus, from an ecotoxicological viewpoint, we conclude that ozonation may improve wastewater quality, which should translate into positive ecological outcomes in the receiving waters. However, because ozonation also can cause toxic transformation products, the process may best be considered on a case-by-case basis.