The present study was carried out to evaluate the biological effects of chitin synthesis inhibitor, applaud (Buprofezin); on immature stages of mosquito, Culex quinquefasciatus. The study showed that the chitin synthesis inhibitor applaud caused biological effects represented in molting failure of larva to subsequent phase or to pupal stage, pupa to adults and mortality. The percentage of the effects differed depending on concentrations used; significantly increased with increasing of concentrations. Also the failure of molting significantly increased compared with mortality at low concentrations and inversely at high concentrations, due to lethal effect of buprofezin at high concentrations. The study of the accumulative death of the first and late inster of larva positively correlated with the time (days after treatment), in addition, the proportion of mortality increased in pupal stage compared with larval stage, also the result indicated that the inhibition of adults emergence increased with concentrations, reached to 95.5% at the highest concentration (1 mg/ L.).