The instar preference and parasitization (expressed as mummification rate) of Aphis gossypii Glover and Myzus persicae (Sulzer) (Hemiptera: Aphididae) by the parasitoid Aphidius colemani Viereck (Hymenoptera: Aphidiidae) were studied at 25 [plus or minus] 0.5[degree]C, 65 [plus or minus] 5% r.h. and a 16L : 8D photoperiod. The female parasitoids were 24-36 h old and were left to forage for I h on an eggplant leaf on which 10 nymphs of each instar of A. gossypii or M persicae were placed. The percentage of A. gossypii nymphs mummified was higher than that of M persicae (43.2 and 25.2%, respectively). The parasitoid parasitized nymphs of all instars of both aphid species, but it showed a preference for 1st and 2nd instars of A. gossypii and 1st instar of M persicae. Nymphs of both aphid species parasitized in the 1st and 2nd instars were mummified when reached the 4th instar or adult stage, whereas those parasitized in the 3rd and 4th instars were mummified in the adult stage. The importance of these results in the effectiveness of A. colemani in biological control of A. gossypii and M persicae is discussed. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts Aphidius colemani (Aphidiidae); Hemipteran hosts; Aphis gossypii & Myzus persicae; Host instar preference, prevalence & host mummification stage Aphis gossypii; Myzus persicae (Aphidoidea ); Hymenopteran parasites; Aphidius colemani; Host instar preference, prevalence & host mummification stage none