The parasitoid wasp Aphidius ervi (Haliday) disrupts formation of wings by its host, the pea aphid, Acyrthosiphon pisum (Harris). Because apterous aphids are larger than alates, we hypothesized that blocking wing formation leads to larger parasitoids emerging from apterous hosts. In an initial experiment, we found that parasitoid larvae could not always prevent host wing formation when the parasitoid began development in fourth instar hosts, allowing us to experimentally test the hypothesis. Both male and female parasitoids emerging from hosts that did not develop wings were heavier than those emerging from alate hosts. Parasitoids emerging from apterous mummies also had significantly longer thoraxes. For insect parasitoids, increasing size has been positively correlated with several measures of fitness. Thus, A. ervi likely gains a fitness advantage by disrupting host wing development.