The predacious midge, Aphidoletes aphidimyza (Rondani), is an effective exclusive natural enemy of aphids in greenhouses and field crops. In this study, prey preference of A. aphidimyza was determined using seven treatments including different stages (third-instar nymphs and adults) of two prey species (Aphis gossypii Glover and Myzus persicae (Sulzer)) based on Manly's β preference index. Results of experiments consisting of two different preys showed that A. aphidimyza larvae consumed A. gossypii more than M. persicae, and the third nymphal stage of both species was preferred more, compared with adults. When all four types of prey were studied, Manly's index was 0.379, 0.235, 0.208, and 0.176 for nymphs of A. gossypii and M. persicae and adults of A. gossypii and M. persicae, respectively. Therefore, the nymphs of both species, as the most preferred prey, were used to study the reproductive performance of the predator in microcosm conditions. The significantly longer adult longevity (female: 7.62 ± 0.15, male: 7.42 ± 0.23 d), higher fecundity (93.75 ± 2.94 offspring per female), and higher intrinsic rate of increase (0.175 ± 0.009 d^-1) of A. aphidimyza were obtained, while consuming third-instar nymphs of A. gossypii. Finally, this study indicated that third-instar nymphs of A. gossypii are the most suitable food for mass rearing of A. aphidimyza.