This work evaluated the biological effects of three chitin synthesis inhibitors; Novaluron (Roxy), Teflubenzuron (Nomolt) and Hexaflumuron (Consult) against the second larval instar of Culex pipiens at different concentrations (0.000001, 0.00001, 0.0001, 0.001 & 0.01 ppm). The results showed that all three tested CSI's were toxic to the larvae as effective dose dependant. Novaluron was more effective followed by teflubenzuron and hexaflumuron. The toxicity index was
100, 50 & 25 with Novaluron, teflubenzuron and hexaflumuron, respectively. All tested CSI's induced a significant prolongation in larval and pupal duration. Also, pupation rate decreased compared with control as well as reduction in adult emergence.