The EU Waterframework Directive aims to achieve a good ecological and chemical status of European surface waters until 2015 and includes monitoring and measures to maintain and improve water quality towards this ambitious aim. Uptil now, water authorities are not obligated to apply in situ ecotoxicity tests for risk assessment. GamTox in situ test was validated to detect and monitor the toxic potential of effluents from waste water treatment plants in two streams, with differences in land use, composition of micro-pollutants and with different species of gammarids as indicators. In both case studies (4 weeks) gammarids reacted already within 1-2 weeks with reduced survival and feeding when exposed in cages below the waste water effluent compared to upstream exposure. Survival appeared to be the more reliable endpoint, as feeding data were variable and gammarids might switch to other food sources which might not directly be related to a toxic effect but to differences in food supply at the exposure location. GamTox has shown potential to be applied for monitoring ecotoxicological water quality below waste water treatment plants.