Organochlorines are the most successful, profitably utilized and commercialized group of pesticides. They have gained huge popularity and prominence in a short span of time by virtue of their ability to control almost all kinds of pests including insect, fungi, rodent, etc. The toxicity of an individual pesticide to the pests is predominantly determined by its structure, the different moieties attached to parent compound, their spatial arrangements within molecule, nature of substituents, polarity, symmetry and asymmetry of molecules, the solubility and sorption values. The present paper discusses the toxicity in terms of LD50 of organochlorine pesticides on the basis of their structures. Further, the mode of action of these pesticides has been discussed for a better understanding of toxicity. Finally an attempt has been made to understand the structure toxicity relationship in organochlorine pesticides. (C) 2006 Elsevier B.V. All rights reserved.