We examined the influence of offspring mortality caused by hyperparasitism on the secondary sex ratio of Lysiphlebus hirticornis Mackauer, a solitary endoparasitoid of the aphid Metopeurum fuscoviride Stroyan, in the field. Females of L. hirticornis produce pseudo-gregarious broods, which may comprise more than 200 offspring. Hyperparasitoids (mainly Syrphophagus aphidivorus (Mayr)) attacked and killed up to 60% of the primary parasitoids inside mummified aphids, especially late in the season. Hyperparasitized broods were larger than hyperparasitoid-free broods, which suggests that the risk of hyperparasitism increased with mummy density. We tested the hypothesis that mortality caused by hyperparasitism is greater for female than male offspring of L. hirticornis. If mummy quality scales with mummy size, hyperparasitoids should choose the relatively larger over the relatively smaller mummies. In the absence of hyperparasitism, broods of L. hirticornis included approximately two daughters for each son;, the sex ratio did not vary with brood size. In hyperparasitized broods, the sex ratio was nearly even. This result indicates that relatively more female offspring (developing in the larger mummies) than male offspring (developing in the smaller mummies) were killed by hyperparasitoids. We propose that sex-differential offspring mortality in L. hirticornis is the result of differences in optimal host choice between the primary parasitoid and the hyperparasitoids. Lysiphlebus hirticornis (Aphidiidae) : Host, Parasite Syrphophagus aphidivorus (Encyrtidae) : Parasite Metopeurum fuscoviride (Aphidoidea) : Host Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Ecology; Population dynamics; Population structure; Land zones; Palaearctic region; Eurasia; Europe Lysiphlebus hirticornis; Hymenopteran parasites; Hyperparasitism effects on sex ratio; Syrphophagus aphidivorus; Hemipteran hosts; Metopeurum fuscoviride; Hyperparasitism; Syrphophagus aphidivorus (Hymenoptera); Population sex ratio; Germany; Bayreuth Metopeurum fuscoviride; Hymenopteran parasites; Hyperparasitism effects on parasite sex ratio; Lysiphlebus & Syrphophagus; Hyperparasitism; Germany; Bayreuth Syrphophagus aphidivorus; Hymenopteran hosts; Hyperparasitism effects on host sex ratio; Lysiphlebus hirticornis; Germany; Bayreuth none