Larval and pupal developments of the malaria vector mosquito, Anopheles stephensi, reared on two different kinds of artificial diets were compared. One diet known to be suitable for larval-pupal and pupal-adult developments shows a normal development in the larval and pupal stages but the other inhibited the development of all stages except for the larval-larval development. From the analysis in the fatty acid profiles of these diets, we found the former diet to be rich in eicosapentaenoic acid (EPA) and the latter to be rich in α-linolenic acid but to be lacking EPA. After addition of EPA to the latter diet, the larvae in which pupal development had been arrested were able to develop to pupae and then adults. Each fatty acid profile of larvae, pupae and adults reared on the individual diets was examined. The main fatty acid compositions of mosquitoes reared on the former diet were palmitic, palmitoleic, stearic, oleic, linoleic, arachidonic and EPA. EPA content in adults (13.7%) was higher than in larvae (7.9%) and pupae (8.6%), and 3.7% in triacylglycerols and 21.1% in phospholipids. Larvae reared on the latter diet showed fatty acid contents of palmitic (30.0%), palmitoleic (22.5%), oleic (17.4%), linoleic (15.9%) and a-linolenic acids (6.1%). From these results we conclude that EPA is an essential fatty acid for larval-pupal and pupal-adult development and that the EPA synthetic pathway does not start from a-linolenic acid in An. stephensi larvae.