none The host-range of two Hymenoptera Braconidae Aphidiinae, Lysiphlebus testaceipes (Cr.) and Lysiphlebus fabarum (Marsh.), polyphagous on aphids of genus Aphis L., has been investigated with particular reference to Toxoptera aurantii B.d.F. and Aphis citricola v.d.G, injurious in Italian citrus orchards. Study of their parasitic activity has led to the demonstration of biotypes on specific aphids. In the case of L. fabarum , a biotype which was active on A. fabae Scop., A. craccivora Koch., and A. hederae Kalt., did not show to be able to complete parasitization of A. citricola, A. pomi De G., A. nerii B.d.F. In the case of L. testaceipes , the biotype differentiation led to the presence of biotypes specialized not only on aphids but even on determined host plants.