Larvae of Tiphodytes gerriphagus Marchal, a parasitoid of water strider eggs, are able to develop within Limnoporus dissortis Drake & Harris eggs that have undergone >80% of their embryological development. In laboratory studies, parasitoid development time was unaffected when young and intermediate aged eggs were attacked, but increased when old host eggs (6 to 8 d) were used. Likewise, wasp body size did not change significantly except for the oldest host eggs, from which significantly smaller wasps emerged. Old egg batches produced higher sex ratios (more males) than young eggs. This resulted from a disproportionate reduction in female eggs laid in the context of a marked overall reduction in parasitoid oviposition. Ability to parasitize over a wide range of host ages likely contributes to the high rates of parasitism observed in the field.