The effect of cypermethrin insecticide in an aquarium fish Poecilia latipinna commonly known as black molly was studied. The samples of 150 fishes were collected from Kolathur of Chennai. Behavioral responses were observed in the control group of fishes which exhibits normal swimming activity and normal opercula movements. Fishes exposed to 0.2, 0.4, 0.6, 0.8 and 1 ppm of cypermethrin noticed many changes in the behavioral response loss of balancing, increased opercula beats, erratic swimming, swimming at the water surface and gulping for air noticed in the experimental fish. The 96 hr LC₅₀ value of the fish exposed to various concentrations of cypermethrin ranging from 0.2 to 1 ppm was plotted and the LC₅₀ values found to be 0.7 ppm. The histopathological changes observed in the liver of Poecilia latipinna exposed to 0.175 ppm concentration at 24 hours, 96 hours, 240 hours and 360 hours includes organization of the hepatic cell, completely vacuolated nuclei, more number of vacuolation in heptocytes, decrease in size of nucleus (Pycnotic).The histomorphology of the intestine in the control fish exhibited simple columnar epithelium, lamina propria, sub mucosa, tunica muscular is and serosa. A few goblet cells containing lymphocytes were located between the epithelial cells. The mucosa was highly branched and folded. The intestine of the experimental fish revealed damage to the appearance of few goblet cells, disintegration of cell wall, and shrinkage of columnar epithelial cells, Damage of columnar epithelial cells and hyperactivity and appearance of more number of goblet cells. The histopathological changes of the liver and intestine were studied in control and experimental fishes using Haematoxylin and Eosin based on the observation, the effect of cypermethrin was discussed with light of the available literature.