none Sex ratio theory has assumed that females can produce offspring of both sexes. It has been suggested that some females in haplodiploid populations are only able to produce sons (constrained sex allocation), for example because they are virgin. The presence of such females influences the optimal sex ratio of unconstrained females. The relevance of these ideas to field sex ratios is largely untested. The frequencies of constrained oviposition in three Drosophila parasitoid species are estimated. Constrained, ovipositing females were distinguished by the absence of sperm in the spermatheca. Constrained females were absent or rate in these species. We review data from the literature that allow an estimate of the frequency of constrained females. We conclude that the available evidence suggests that while constrained oviposition is uncommon, there are some species in which constrained females are sufficiently common to select for an observable sex ratio bias by unconstrained females. Reproduction; Reproductive behaviour; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Land and freshwater zones; Palaearctic region; Europe; United Kingdom Drosophila (Drosophilidae); Hymenopteran parasites, survey; Host parasite relationships; Hymenoptera; England; Silwood Park, constrained sex allocation Hymenoptera; Sex determination; Oviposition; Constrained sex allocation; England & review Asobara tabida; Tanycarpa punctata (Braconidae ); Leptopilina heterotoma (Eucoilidae ); Dipteran hosts; Drosophila; Host parasite relationships; Drosophila (Diptera); England; Silwood Park; Constrained sex allocation, survey