Parasitoid assemblages associated with Phyllonorycter spp. leafminers (Lepidoptera: Gracillariidae) were examined on six deciduous oak species (Quercus, Fagaceae) in Japan to understand how host leafminers, host food plants, and geographic location influence the structure and function of parasitoid assemblages. The parasitoid assemblages on five of the six oak species showed similar species richness (18–20 spp.), regardless of the number of host leafminer species and the abundance of hosts. Species composition was influenced by geographic location as well as host food oaks, and to a lesser extent by the leafminers. The assemblages varied in guild structure but showed nearly equal ratios of koinobiont/idiobiont species. The ratio was almost the same as that in Great Britain. These results suggest that the well-known argument that parasitoid communities are mostly influenced by host food plants is not universal, and that parasitoid assemblages may be organized with a balance between numbers of koinobiont and idiobiont species regardless of species richness, species composition, or geographic location. Total rate of parasitism did not correlate with the number of parasitoid species or the evenness of the assemblage. This implies that the host–parasitoid system is not under top-down control.