Ephialtes hokkaidonis was found to be a parasitoid of a mud wasp, Symmorphus sp.(Hymenoptera, Vespidae). Examination of Symmorphus nest that was infested by E. hokkaidonis revealed that the females lay an egg by penetrating the mud end plug of host nest and its larva grew up as an external idiobiont. Observed oviposition behavior of the parasitoid consisted of three stages, I) detecting the host nest, 2) ovipositor locating, and 3) penetrating and laying eggs. When penetrating the end plug, ovipositor tip seemed not so strongly pressed against the plug, but a tiny hole was made and it was spread by the back and forth movement of small teeth of lower valves of ovipositor. E. hokkaidonis is unique in the structure of ovipositor tip, as it is cylindrical and somewhat depressed, with its lower valve forming the large dorsal lobe which is equipped with close teeth and almost encloses upper valve from the tip to the basal tooth. Recorded hosts of the genus Ephialtes were reviewed and divided into two groups, woodborers and tube nesting Aculeate. Based on the oviposition behavior and the structure of ovipositor, the regular host of Ephialtes hokkaidonis and presumably its congeners seemed to be an Aculeate Hymenoptera. Immature stages and unknown male of this species were also described. Japan-, Honshu-, Hyogo none