We studied interactions between males and females of the Australian pollinating fig wasp, Pleistodontes imperialis (Chalcidoidea, Agaonidae), in Ficus platypoda (Moraceae). As for many other fig wasps, all mating occurs within the confines of a syconium before females depart: We show that initially there is scramble competition between males for access to virgin females. During this time males excavated a small hole into a female's gall to mate through. These holes were just large enough for insemination, but not large enough for females to exit their galls. Males ignored mated females, and as virgin females became scarce males switched strategies and began to enlarge insemination holes until they were large enough for females to escape, showing that males enhance female fitness by means other than just mating. Syconia with experimentally reduced numbers of males had fewer liberated females, suggesting that female fitness is strongly affected by the number of males present. Females may be unable to escape their galls unassisted because of morphological adaptations to syconium founding. We argue that sex allocation should be affected not only by competition among males but also by intersexual interactions between siblings. This could potentially offset the strong female bias predicted bY local mate competition. (C) 2000 The Association for the Study of Animal Behaviour.