Abstract: Present study was aimed at investigating the toxic effects on the eleven day-old-chick embryo of different concentrations (50, 100, 200 and 400 ppm) of cypermethrin insecticide administered as a single sublethal dose (0.05 ml) into the eggs at day ‘0’ of incubation. Toxicity was evaluated in terms of teratological and biochemical changes in the developing embryo. Among biochemical constituents, activities of a few enzymes and some biochemical contents of the whole embryo were investigated. AST, ALT and LDH activities were affected with cypermethrin treatment. Of the transaminases, the activity of AST was increased whereas, the activity of ALT decreased. Activity of AST was increased at 100 ppm, while the activity of ALT was decreased at 200 and 400 ppm. LDH activity was decreased at 100 ppm. The activities of Amylase AkP and AcP remained unaltered with cypermethrin treatment. Cypermethrin treatment has resulted in the decrease of embryonic glucose, soluble protein and free amino acids contents and increase in the cholesterol and urea contents. Glucose content was decreased at 100, 200 and 400 ppm and soluble protein and free amino acid contents were decreased at 200 and 400 ppm. Cholesterol was increased at 400 ppm. Urea content was increased at 200 and 400 ppm. RNA showed a mixed response, it was increased at the dose of 50 ppm and decreased at 100 ppm. Glycogen, total protein, total lipids, uric acid and DNA contents remained unchanged with cypermethrin treatment. No malformations were observed at the dose of 50 ppm of cypermethrin treatment in eleven-day-old chick embryo However, at the doses of 100, 200 and 400 ppm teratological changes were observed, which included the reduction in crown rump length and in the size of eye ball, incomplete development of beak and totally absence of beak (at some doses), lower jaw, legs, wings and feathers. Crown rump length was reduced at the doses of 100, 200 and 400 ppm of cypermethrin. At the dose of 100 ppm, eye was not developed, however, at 200 and 400 ppm there was significant reduction in the size of eyeballs. At 100 ppm beak was incompletely developed whereas, lower jaw was totally absent. At the doses of 200 and 400 ppm beak, legs, wings and feathers were not developed. All these data suggest that cypermethrin is extremely toxic to developing organisms.