The behaviour and development of Elasmosoma sp., near pergandei Ashm. (Neoneurinae: Braconidae: Hymenoptera), a parasite of the ant, Formica obscuriventris clivia Creighton, was studied in a coastal sand dune habitat in central Oregon over a 2-year period. Female wasps oviposited into the abdomen of major worker ants after alighting briefly on the host. Embryonic development of the wasp occurred inside the serosal membrane or trophamnion of the egg in the abdomen of the ant host. First instar wasp larvae possessed a large head Capsule with falcate mandibles, 11 ventral body lobes, an elongate tail and a dorsally located anus. Second instar larvae were unspecialized and lacked the large head capsule, elongate tail and paired ventral body lobes. Third instar larvae were hymenopteriform with a distinct head capsule. Cocoon formation and adult emergence is described. Observations on the percentage parasitism and altered bchaviour of parasitized ants are presented. The association between neoneurine wasps and ants is a well-balanced system that has been in existence for at least 40 million years. Life cycle and development; Development; Behaviour; Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Habitat; Terrestrial habitat; Land zones; Nearctic region; USA; North America Elasmosoma (Braconidae); Embryo development; Larval development; Behaviour; Hymenopteran hosts; Formica obscuriventris clivia; Parasitoid behaviour & development; Sand dune habitat; Coastal dune habitat; Oregon; Lincoln County; Waldport; Behaviour & development, hymenopteran host, coastal sand dune habitat Formica obscuriventris clivia (Formicidae); Hymenopteran parasites; Elasmosoma; Parasitoid behaviour & development; Sand dune habitat; Coastal dune habitat; Oregon; Lincoln County; Waldport; Hymenopteran parasitoid behaviour & development, coastal sand dune habitat none