Field and experimental observations on the European ichneumonid Sphinctus serotinus as a koinobiont ectoparasitoid of the limacodid moth Apoda limacodes are recorded. The egg is anchored into the extremely thick cuticle of its host but not deeply enough so that it would survive host ecdysis. That may explain the late summer-early autumn flight time of the parasitoid, when practically all host larvae that remain are in their final instar, though avoiding competition from other parasitoids may also play a part. The tough cuticle of the host probably also underlies the lack of host feeding by the adult parasitoid. There is no ability to avoid superparasitism, though self-superparasitism is limited by post-oviposition flight to leave the host. The egg can be laid with its chorion still uncoloured when hosts are in plentiful supply, but such eggs become the usual pale brown colour within a short time. No dumping of eggs in the absence of hosts occurs. The egg normally doesn’t hatch until after the host has prepared its cocoon, and the parasitoid larva, still anchored in the eggshell at first, feeds very slowly through the winter and early summer on the prepupal host. Development through the prepupal and pupal stages within the host cocoon similarly proceed slowly, with no evident diapause at any stage. The various stages of the life cycle are illustrated photographically.