Ecological immunology distinguishes between the long-term evolutionary costs of possessing defences against parasites and the short-term costs of using them. Evolutionary biologists have typically focused on the former in the search for constraints on the evolution of resistance. Here, we show in the peach-potato aphid, Myzus persicae, that short-term costs may be of equal evolutionary importance. Survivors of more resistant aphid clones suffered a higher reduction of fecundity upon parasitoid attack than survivors of more susceptible clones. This genetically based trade-off between benefits and costs of defence may limit the evolution of increased resistance and explain the maintenance of genetic variation for resistance under environmental variation in parasitism risk. Myzus persicae (Aphidoidea): Host; Aphidius colemani (Aphidiidae): Parasite. Aphidius colemani: Hemipteran hosts, Myzus persicae, Host defence costs and effectiveness, Victoria, Bacchus Marsh; Myzus persicae: Fecundity, susceptibility, defence costs and effectiveness, Genotype, Microsatellite genotype, Hymenopteran parasites, Aphidius colemani, Victoria, Bacchus Marsh. Arthropods; Hymenopterans; Insects; Invertebrates; True Bugs. none