none The ability of Lysiphlebus testaceipes (Cresson) females to discriminate between parasitised and unparasitised Aphis gossypii Glover, and the mechanisms of host discrimination was studied. By comparing the resting time of parasitoid females on leaves with parasitised and unparasitised aphids their ability to host discriminate was evidenced. Y-tube olfactometer tests and direct observations of the searching behaviour of L. testaceipes females showed that the markers released during the parasitisation (at the insertion of the ovipositor and during egg laying) are perceived neither over long nor over short distance, but they are probably responsible of contact host discrimination. However, the parasitoids use their antennae to perceive olfactory cues and thus recognise parasitised hosts. The cues that act over short distance are directly or indirectly induced by the egg or the larva of the parasitoid developing inside the host. Sensory reception; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Lysiphlebus testaceipes (Aphidiidae); Chemoreception; Hemipteran hosts; Aphis gossypii; Parasitized vs unparasitized host discrimination mechanisms Aphis gossypii (Aphidoidea); Hymenopteran parasites; Lysiphlebus testaceipes; Parasitized vs unparasitized host discrimination mechanisms