The neem tree Azadirachta indica Juss (Meliaceae) is one of the most studied plant species for pest control, including mosquitoes. However, the effect of aqueous neem seed extracts (ANSE) on each of the 4 instars of mosquito Culex quinquefasciatus Say (Diptera: Culicidae) is unknown. In order to determine the effect of applying ANSE on the growth of each of the 4 larval instars and pupal stage of C. quinquefasciatus, 8 bioassays with ANSE concentrations ranging from 30 to 1000 ppm were applied to larvae of 1st, 2nd, 3rd and 4th instars. The experimental unit consisted of a plastic cup with 100 ml of water and 20 larvae. 1 ml of the different concentrations of ANSE was applied. Mortality was assessed when 93 to 98% of the larvae reached the pupae stage in the control. In the surviving population the duration of larval stages and pupae viability were recorded. The growth of larvae and development of pupae was recorded to determine a growth inhibition index (GII) and a relative growth index (RGI), which was used to calculate the EC50 (concentration of ANSE that caused 50% reduction in RGI) for each larval stage. The concentrations of 460, 440, 410 and 550 ppm inhibited the growth of 50% the population of larvae of 1st, 2nd, 3rd and 4th instars, respectively; concentrations 500 and 700 ppm applied to larvae of 1st and 2nd instar, respectively, prevented the formation of pupae and concentrations 700 ppm prevented the emergence of adults in 4th instar larvae. The results showed that the different treatments inhibited the growth of larvae and pupae, prevented the formation and inhibited the development of pupae and reduced larvae and pupae viability.