Parasitoid assemblages associated withPhyllonorycterleaf miners (Lepidoptera: Gracillariidae) were examined on the 2 deciduous oaks,Quercus dentataandQ. mongolica, in Hokkaido, northern Japan, and those on 2 others,Q. acutissimaandQ. variabilis, in Nara, central Japan. To address to what extent interspecific competition is important in organizing parasitoid communities, I compared species richness, species composition, and levels of parasitism by guilds at different host immature stages among the parasitoid assemblages. Parasitoids were separated into 5 guilds according to parasitism modes (idiobiosis and koinobiosis) and host immature stages attacked and killed. Pooled data showed that the number of parasitoid species per host leafminer species in Japan (3.1) was similar to that in the United Kingdom (4.1) and that idiobionts (potential generalists) exceeded koinobionts (specialists) in species number (62.5%). A koinobiont guild and 2 idiobiont guilds had an inverse relationship in level of parasitism between 2 assemblages at each study area. These results may suggest that interspecific competition is important in organizing parasitoid communities. Nevertheless, dominant species and guilds varied among the assemblages, resulting in different patterns of percentage of parasitism by guilds in relation to host stage among the assemblages. This implies underuse of hosts at some stages by parasitoids, and interspecific competition is unlikely to be severe. Interspecific competition, therefore, seems to partially contribute to parasitoid community organization