Abstract: The water ecological criteria of acetonitrile, which is an important pollutant in the wastewater of acrylonitrileacrylic plants, were studied by conduting bioassay on the basis of features of the aquatic biota in China and with reference to "Guideline for deriving numerical national water quality criteria for the protection of aquatic organism and their uses" of the U. S. A The Criteria Continuous Concentration (CCC) and Criteria Maximum Concentration were calculated from the toxicity data of acute toxicity tests with thirteen animals species, chronic toxicity tests with aquac animals species and plant toxicity test with an aquatic plant Lemna minor. The CCC and CMC for acetonitrile were 413mg/L and 1145 mg/L, respechvely.