The species composition and the population dynamics of aphidophagous syrphids (Diptera: Syrphidae) in polyethylene-covered greenhouses of sweet pepper (Capsicum spp.) were studied over a 3-yr period in southeastern Spain. Modified Malaise traps were used to sample the adult flies within the greenhouses, accompanied by direct sampling of syrphid larvae on the sweet pepper plants. Over a 2-yr period, modified Malaise traps also were used to record the movement of syrphids into the greenhouses from the surrounding environment. Additionally, the effect of opening or closing the side walls of the greenhouse (ventilation management) on syrphids was assessed by census techniques. In total, nine species of aphidophagous syrphids were found as larvae, preying on aphids on sweet pepper plants. Three of these species, Eupeodes corollae (F.), Episyrphus balteatus (De Geer), and Sphaerophoria rueppellii (Wiedemann), made up 98% of the larvae collected. There was a temporal succession during the season, attributed to temperature, and variability among years related to drought. S. rueppellii seems to be the most adapted to survival in the high temperatures and dry conditions. Modified Malaise traps proved effective for monitoring adult syrphids in greenhouses, but they also captured species whose larvae are not associated with the aphid colonies in sweet pepper. This indicates that when studying aphidophagous syrphids, sampling of adults by Malaise trap should be accompanied by sampling of larvae. Larger numbers of syrphids were captured outside of the greenhouse than inside. Opening the side walls of greenhouses increased syrphid numbers within it, and this should be considered in pest management regimes.