A static acute 120-hour EC50 study of 5% Peracetic Acid (Vigor Ox) was conducted in the freshwater green alga, Selenastrwn capricornuturn. Nominal concentrations of 0.065, 0.13, 0.25, 0.50 and 1.0 mg/L were prepared in 100 m1s of algal medium in triplicate. All flasks were inoculated with approximately 3,000 cells/mL. All flasks were placed on a rotary shaker and incubated for 120 hours at 24 degrees centigrade under cool-white fluorescent lighting. Temperature and light intensity were monitored and recorded daily throughout the study. The pH of the test solutions was recorded at 0 and 120 hours. Test concentrations were measured at 0 and 120-hours using a residue method involving spectrophotometric analysis. At 0-hour, measured concentrations averaged 98% of nominal. At 120-hours, measured concentrations from test levels 1-3 resulted in values below the minimum quantifiable limit (