Novaluron is a chitin synthesis inhibitor belonging to the class of benzoylphenylurea. A commercial formulation of novaluron (MCW-275) was tested at different concentrations, ranging between 0.2 and 1.6μg/l, against third and fourth-instar larvae of Culex pipiens. Treatments were done on newly moulted larvae for 24 h (WHO) under standard laboratory conditions. Mortality was observed for both stages, after incomplete development and the LC50 and LC90 were estimated at 0.32 and 1.2 μg/l for the third-instar larvae, respectively, while these respective values were 0.58 and 2.2μg/l for the fourth-instar larvae. Furthermore, the compound was applied at LC50 and LC90 against the fourth instars larvae and its effects was investigated on development duration, biochemical composition of larval body and body weight. Metabolite analyzes showed that novaluron affected significantly the amount of carbohydrates, lipids and proteins of fourth instars larvae starting day three following treatment. For the same treated series a significant decrease was also recorded in the body weight with an increase of development duration. The final step of the chitin biosynthesis pathway is inhibited by the novaluron and the precursor is not converted in to chitin and this leads to death or the increase in life time of the larvae.