Polemon liparae Gir. is an important parasite of the chloropid fly Lipara lucens Mg. which induces the formation of a gall on Common Reed (Phragmites communis Trin.). The females of Polemon search for the eggs of Lipara, that are attached to the leaves and stem of the plant. They walk up and down the reed culms with the antennae touching as great a part of the surface of the plant as possible. When an egg is found the insect shifts its position in such a way that the mouthparts touch the far end of the spindle-shaped egg. Next the abdomen is bent forward under the thorax and the egg shell is stroked by the ovipositor. The stroking may be followed by oviposition. This always occurs at the end of the egg. It is suggested that the stroking serves egg recognition and the distinction of parasitized from unparasitized eggs. Contacts with unparasitized eggs lead to oviposition in a much greater proportion of cases than those with eggs that are already parasitized. Only about one half of an egg is tested at a time in this way. The result is that, under laboratory conditions, some host eggs are parasitized two times, receiving a parasite egg at either end. Polemon is an egg larval parasite.