In some insect species, the presence of a mate at the time of eclosion appears to facilitate rapid mating, with positive fitness consequences for one or both mates. Field observations that males of the hymenopteran parasitoid Pimpla disparis Viereck aggregated on a gypsy moth, Lymantria dispar (L.), host pupa before the emergence of a female led us to hypothesize that these males responded to chemical cues associated with parasitized host pupae. Results of laboratory experiments with wax moth, Galleria mellonella (L.), host pupae suggest that female P. disparis chemically mark the host pupae they have parasitized and that males discern between such pupae and those not parasitized. As males continue to recognize parasitized host pupae throughout the development of the parasitoid, they could exploit not only the females' marker pheromone but possibly also semiochemical, visual, or vibratory cues from the developing parasitoid inside the host pupa, the decaying host, or both. Irrespective, these cues could help males locate parasitized host pupae and time the emergence of a prospective mate.