none Host finding by the parasitoid Ascogaster quadridentatus (Wemael), a braconid wasp, is mediated by the kairomone present in the eggs of the host, the coding moth Laspeyresia pomonella L. A very stabile contact chemical stimulus in the host eggs elicited an antennal response and oviposition-behavior but no oviposition, characteristic of adult wasps. It seems that after oviposition the host egg is marked by the wasp with a repellent-substance to prevent a second oviposition by another wasp. The kairomone is attractive to wasps in very low concentrations (3/1000 parts of an egg dissolved in methanol induced significant oviposition-behavior). Waterly and methanolic host egg-extracts are significantly more attractive to the wasps than chloroformic, benzolic and ethanolic extracts. After precleaning of the methanolic extract by column-chromatography the kairmone can be detected on a thin-layer silica-gel plate as a small spot by the wasps themselves. The plates were developed in methanol (normal saturation of the developing box, without paper) and the h.R.f. of the attractive kairomone-areal was 74 +- 3. Therefore it seems that the kairomone obtains only one substance.