Spraying of various pesticides for controlling pests has been a very common affair in agricultural practice. As the pests develop resistance to the pesticides, use of stronger pestcides become inevitable. These pesticides accumulate in the environment and cause serious hazards to biosphere. The present study was carried to elaborate the toxic effect of one such pesticide (Endosulfan) on the fresh water. Mesocyclops leuckarti (Claus, 1857) was selected as a model for studying the effects of Endosulfan. Endosulfanis known as a highly efficient pesticide. At the same time details discussed in the following paper will prove that its effects on the aquatic microfauna are also significantly toxic, LC50 studies of the same have indicated that even minute doses like 0.01 ppm were lethal. For M.leuckarti when exposure time was 96 hours. The forthcoming paper details LC50 values of this pesticide for different periods of exposure.