Purpose: Contaminated sediments are an important exposure pathway for the aquatic fauna in the Rhine River. We applied bioassays with the aim to characterize the ecotoxicological hazard potential of sediments of an oxbow lake of the Rhine River, especially to fish. Potential effects on fish and water flea were evaluated indirectly by applying in vitro and in vivo bioassays in the laboratory. Results were compared with those of the official German risk assessment of dredged sediments. Materials and methods: Sediments taken from 13 sites along a 600-m transect line were tested for acute toxicity to water flea (Daphnia magna immobilization test), teratogenicity, and embryotoxicity (sediment contact test with Danio rerio), as well as for cytotoxicity (neutral red retention assay with RTL-W1 cells) and estrogenic effects (lyticase-assisted yeast estrogen screen (L-YES) assay). The tests were conducted using pore water, organic extracts, or native sediments. Spatial patterns of the measured effects were also assessed. Results and discussion: Virtually all samples induced estrogenic, teratogenic, embryotoxic, and cytotoxic effects, but no acute toxicity on D. magna was observed. Cytotoxicity was in accordance with previous studies on the Rhine, Neckar, and Danube Rivers. Estrogenic effects were in the range of estradiol equivalent (EEQ) values detected in UK estuaries. Although sediment contact tests with D. rerio embryos showed virtually no mortality, sublethal effects were common. Some of the effects increased with increasing distance to the main channel. Conclusions: The test with D. magna is, along with bacteria and algae toxicity assays, an important part of the German standard risk assessment for sediments. However, it failed to identify the ecological hazard of our sediment samples to fish. Our results indicate that adverse effects on fish are possible and suggest the need for revising risk assessment procedures in order to address the risk for this important organism group in aquatic ecosystems. ; Fil: Schulze Sylvester, Maria. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Instituto Para El Estudio de la Biodiversidad de Invertebrados; Argentina. Universitat Koblenz. Landau Institut Fur Biologie; Alemania. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Salta; Argentina ; Fil: Heimann, Wilko. Universitat Koblenz. Landau Institut Fur Biologie; Alemania ; Fil: Maletz, Sibylle. Rwth Aachen University, Institute For Biology V; Alemania ; Fil: Seiler, Thomas Benjamin. Rwth Aachen University, Institute For Biology V; Alemania ; Fil: Brinkmann, Markus. Rwth Aachen University, Institute For Biology V; Alemania ; Fil: Zielke, Hanno. Rwth Aachen University, Institute For Biology V; Alemania ; Fil: Schulz, Ralf. Universitat Koblenz. Landau Institut Fur Biologie; Alemania ; Fil: Hollert, Henner. Rwth Aachen University, Institute For Biology V; Alemania. Chongqing University, College of Resources and Environmental Science; China. Tongji University, College of Environmental Science and Engineering and State Key Laboratory of Pollution Control and Resource Reuse; China. Nanjing University, State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment; China