A variety of agri-environmental instruments can be used to combat water pollution by pesticides in agriculture according to the regulation EC 1257/1999. The chosen policies should be politically feasible and capable to prevent external effects cost-effectively. One possible policy instrument for Germany is the implementation of nation-wide buffer strip programs. This paper aims at evaluating the cost-effectiveness of alternative strip widths. Therefore an indicator is developed that reflects the impact of plant protectant use on aquatic ecosystems. This indicator is used to compute reductions in pesticide impact of different strip widths and the agricultural opportunity costs computed by the agri-environmental policy-information-system RAUMIS.