Abstract: Background and Objective: Acetamiprid is a neonicotinoid pesticide and it is used by the farmers to protect their crops. This pesticide residue reaches to the aquatic ecosystem by many ways and affects the aquatic fauna. The aim of the study was to analyze the effect of pesticide-acetamiprid on electrophoretic protein patterns of liver, brain and gill tissues of Oreochromis mossambicus (O. mossambicus). Materials and Methods: Lethal concentration (LC50) of acetamiprid for O. mossambicus has been calculated by probit analysis. Sublethal or safe level concentrations were derived from 96 h LC50. In the present study 1/15 of the 96 h LC50 were selected as sublethal concentration and the fishes were exposed to this concentration for a period of 7 and 14 days. Results: The electrophoretogram of 7th and 14th day exposure represents the decrease in the intensity of liver, brain and gill protein subunits and increase in protein bands due to acetamiprid toxicity, when compared to control. Studies revealed that due to the toxic effect of the pesticides the protein bands get denatured, as a result there was a decrease in the intensity of protein bands and there was an increase in new protein bands due to the enhancement of stress proteins. Conclusion: The alterations in electrophoretic protein patterns observed in various tissues of the experimental fish could serve as sensitive biochemical indicators of acetamiprid pollution, which might help in aquatic management. It may be concluded that long term exposure to acetamiprid creates a continuous health hazard for Oreochromis mossambicus.