A 48-hour static acute toxicity test with 5% peracetic acid was conducted in Daphnia magna from 12-14 of February 1996. Dilutions of a freshly prepared primary stock of 5% peracetic acid was used to obtain nominal concentrations of 0.19, 0.32, 0.54, 0.90 and 1.5 mg/L. Duplicates of each test concentration and control were established. Twenty daphnids were used per treatment. Test solutions were analyzed on days 0 and 48 using a spectrophotometric method. Overall, the measured concentrations represented 100% of the nominal concentrations. Observations of immobility, sub- lethal effects or other overt signs of toxicity were made daily. Water quality parameters such as temperature, pH, and dissolved oxygen were recorded at prescribed intervals. After 48-hours of exposure, 90% immobility was observed among daphnids exposed 0.86 mg/L and 100% immobility was observed among daphnids exposed to 1.4 mg/L. No effects were observed at the lower concentrations. The 48-hout EC50 was 0.73 mg/L and the 48-hour NOEC was 0.56 mg/L. Based on the results of this study, 5% peracetic acid is toxic to daphnia magna.