AbstractA quantitative structure—activity relationship study is carried out with use of the toxicity (LC50) of the various kinds of fish obtained from the references. The equation obtained contained the parameters of molecular weight (MW) and mean ionization potential (IP):log(1/LC50) = 28.665 + 0.01095MW ‐ 2.570IP(R2= 0.695,n= 581). This equation could estimate the toxicity of the chemicals detected in the environment of Japan by Environment Agency (r= 0.80,n= 53). In order to evaluate the hazardousness, we propose the Ecotoxicological Risk Quotient (ERQ):ERQc= ‐log Concentration in the environment/Effective concentration in the testERQa= ‐log Σ Concentration in the environment/Effective concentration in the testERQcand ERQaare the indices of the hazardousness of a chemical and chemicals in an area, respectively. ERQ could explain the status of pollution in water environment