none The host acceptance behavior of the Japanese aquatic wasp, Agriotypus gracilis Waterston, an ectoparasitoid of the sand case building caddisfly, Goera japonica Banks was investigated in the laboratory. Female wasps were observed to enter the water by walking down a stone protruding from the water surface. Antennae were held backward and not utilized in searching for hosts under the water. Female wasps examined hosts from the outside of their cases by 2 consecutive steps, "turning" and "probing". Turning behavior, in which female wasps move between the anterior and posterior ends of host cases, may be related to the measurement of case size. Host stages are considered to be discriminated by probing, in which females probe host cases with their sheathing ovipositors. Female wasps most frequently accepted and oviposited on pupal and prepupal hosts.