The occurrence of Glyphipterix schoenicolella (Lepidoptera, Glyphipterigidae) in dune slacks of The Netherlands is investigated by collecting 20 infructescences per subpopulation of Black Bog-rush (Schoenus nigricans) in plastic bottles from early May onwards. The larvae of G. schoenicolella are monophageous in the developing infructescences of Black Bog-rush. The life cycle of the insects was also studied in five microcosms. Between March and May, the first-instar larvae are leaving the eggs being deposited in the previous autumn at the basis of the tussocks with completely underdeveloped inflorescences. The neonate larvae have to crawl along the stem (20 to 40 cm) up to the inflorescence to reach their food, i.e., very young nutlets that are only present in position 3 and 4 of each spikelet. Third-stadium larvae have sufficiently strong mandibles to penetrate from the insertion point of the style into the silicified nutlets. Prior to pupation a silky cocoon is establish! ed, its outer surface strongly covered by larval faeces. After a period of 7-10 days the moths hatch and start sucking nectar and consuming pollen grains from Mentha aquatica prior to laying eggs. The degree of infestation of infructescences of the Black Bog-rush varies between sites and years, mostly one, but seldom three insects per infructescence. By multiplying the number of specimens per infructescence with the number of infructescences per tussock (25 to 500) and the number of plants of vital populations (50 to 400), the local population of G. schoenicolella is estimated between 50 and some thousands; but only once more than 60 moths were observed simultaneously. The flying period of the moths starts in June with a peak in July and early August. Diadegma sordipes (Ichneumonidea, Camplopleginae) as parasitoid of G. schoenicolella hatches nearly a week later than the first specimen of the moth. Only once another parasitoid belonging to the subfamily Crytinae was found. ! Cryptinae (Ichneumonidae): Parasite; Diadegma sordipes (Ichneumonidae): Parasite; Glyphipterix schoenicolella (Glyphipterigidae): Host. Cryptinae, Diadegma sordipes: Lepidopteran hosts, Glyphipterix schoenicolella, Recorded, Netherlands; Glyphipterix schoenicolella: Food plants, Schoenus nigricans, Life cycle, Hymenopteran parasites, Diadegma sordipes, Recorded, Cryptinae, Ecology, Sand dune habitat, Netherlands. ST Animalia, Arthropoda, Insecta, Hymenoptera, Apocrita, Terebrantia, Ichneumonoidea, Ichneumonidae; Animalia, Arthropoda, Insecta, Lepidoptera, Glossata, Heteroneura, Yponomeutoidea, Glyphipterigidae. Arthropods; Hymenopterans; Insects; Invertebrates; Lepidopterans. none