This report presents several experiments carried out in a study to develop standardized sediment toxicity tests. The prolonged toxicity of chlorpyriphos (CPF) to the waterflea Daphnia magna was determined in a sediment/water-system, an elutriate and in Dutch Standard Water. The toxicity of CPF could be established in the elutriate and in Dutch Standard Water. NOEC values were 0,32 mg/kg dw and 0,032 mug/l respectively (concentrations as nominal values). The results in the sediment/water-system were unreliable due to low reproduction and growth of the daphnids during the 21 day exposure period.