Nowadays, Aquatic pollution has become a global problem. Fishes are sensitive to a wide variety of pesticide chemicals. The effect of Cypermethrin (25% EC) pesticide on behaviour and histopathology of black molly, P. latipinna is studied. The 96hr LC50 concentration is found as 0.035ppm. 1/10th of the concentration 0.0035ppm is used for this study. Decreased swimming activity, excessive secretions of mucous around opercular region are observed in Cypermethrin exposed fishes. The gill histopathological changes observed in at 1st, 4th, 8th, 12th and 16th day of 0.0035ppm exposure exhibited gill lamellae includes less affected and less damages in the gill structures, curling and fusion and shortening of gill lamellae, severe necrosis in primary and secondary gill lamellae, club shaped secondary gill lamellae. Cypermethrin (25% EC) pesticide becomes toxic to the aquatic organisms even in low concentration.