none Samples of the Citrus Aphid Toxoptera aurartiae B.d.F. (Homoptera Aphidoidea) collected in Central and South Italy before and after the accidental introduction of the American endoparasitoid Lysiphlebus testaceipes (Cr.) (Hymenoptera Ichneumonoidea) were compared. L. testaceipes seems to be able to compete successfully with the indigenous parasitoid species thus altering their respective relative abundance in relation to the host aphid. Circumstantial evidence indicates that the original parasitoid spectrum which was characterized by the dominance of the local species Lysiphletus fabarum has been modified by the spontaneous diffusion of L. testaceipes . The latter seems to be able to displace L. fabarum in its original distribution area.