Different species of aphids in rapeseed fields of Khuzestan province were identified and the dominant ones were determined during 2002-2004 field sampling. Base upon the collected data, population fluctuations of rapeseed aphids vs. their natural enemies were assessed. Also biology of the aphids, major species was studied under controlled conditions. The results showed that Lipaphis erysimi (Kalt.), Brevicoryne brassicae (L.) and Myzus persicae (Sulzer) were active in rapeseed fields of Khuzestan with 66.71, 24.99 and 8.29 percent of population, respectively. The contamination index of L.erysimi was minimum in February (10.5–28.6) while maximum (224–1072) in early until mid March at an average temperature of 17-17.6?C and %50 R.H.. The contamination index in 2004 was more than in 2003. Controlled conditions indicated that L. erysimi bears a life cycle of 7.65?1.12 days along with the ability to reproduce 43.5?18.5 nymphs in her life time. The major predators were Cocinella septempunctata L., and Episyrphus balteatus F. which emerged in December and January with their peaks occuring in March. Two parasitoid wasps Diaeretella rapae M. and Aphidius ervi H. with 78.2 and 21.8 percentage of total population respectively, parasitized the aphids with 1.76–8 percents during the studing period. The above natural enemies could not control the increased population of the aphids because of limited fecundity and feeding and as well lack of synchrony with the aphid life cycle.