none The possible causes determining the dominant activity of Lysiphlebus testaceipes (Cresson) in comparison with Lysiphlebus fabarum (Marshall) on the Citrus brown aphid, Toxoptera aurantii , B.d.F., in the Mediterranean area, and on several species of Aphis L., have been examined. The comparison is based on a comparative examination of some important characteristics (longevity, sexual behaviour, oviposition, fecundity of females, sex ratio) of the adults reared on the favourite natural hosts such as Toxoptera aurantii , on tangerine and lemon sprouts, and Aphis fabae Scop., on bean plants. The tests have shown that L. testaceipes , in comparison with L. fabarum , presents higher longevity (and resistance to starvation) of adults, a sex ratio more in favour of females, and rapidity of oviposition.