1 physicochemical properties including the functional groups, (ii) solubility in water and organic solvents, (iii) dose/concentration, (iv) ionic characteristics (v) translocation and biotransformation, (vi) their mode of action, and (vii) interaction with other chemicals. Almost all these characteristics are dependent on the structure of compound. 2 Toxicity of chemicals is greatly affected by various factors pertaining to their exposures to the organisms. 3 The factors of surrounding medium affecting the toxicity of chemicals have been extensively worked out for the aquatic medium. The factors related to the medium are physico- chemical characteristics of the water affecting the toxicity of chemicals 4 the factors related to the organisms are very important in the study of toxicity of xenobiotics.