In some animals, transformation to the adult stage occurs in a hidden location, such as a burrow or a host. Males that can locate hidden females sooner, e.g., before they emerge, may have a mating advantage, particularly if the females are ready to mate. Whether males locate pre-emergent females and whether pre-emergent females will mate was examined in the parasitoid wasp Spalangia endius Walker. S. endius parasitize fly pupae. A single wasp offspring feeds, pupates, and transforms into an adult within the fly puparium (an outer shell around the fly pupa), and males emerge a day or more before females. Whether pre-emergent wasps are ready to mate was examined by dissecting them out of their hosts and presenting them with naturally emerged adults of the opposite sex. Many of the pre-emergent wasps were ready to mate. Nevertheless, males did not distinguish between hosts containing a pre-emergent female versus a pre-emergent male, or even between parasitized and unparasitized hosts. In contrast, males were able to differentiate between hosts from which a female versus a male had recently emerged. Although females are ready to mate before emergence, there may be little advantage to recognizing and staying with a host that contains a pre-emergent female because emergence takes so long, which raises the cost of missed mating opportunities elsewhere.