Observations of M. sexta (L.) larvae parasitized by A. smerinthi Riley indicated that parasitism induced only partial host developmental arrest, since host molting was suppressed during parasitoid emergence but not thereafter. After parasitization of first-stage M. sexta larvae, parasitoid emergence usually occurred during the host's fourth larval stage. In many hosts, cuticle was secreted over the surface of partly emerged parasitoids. The occurrence of post-emergence molting suggests that the host's ecdysteroid titer rises after emergence, in contrast to the low ecdysteroid titers measured after emergence in larvae parasitized by A. congregatus (Say), a highly co-evolved parasitoid. Together with the finding that few A. smerinthi larvae that emerged spun cocoons and eclosed as adults, the observed anomalous effects of parasitism were interpreted as indicative that this species is poorly adapted to using M. sexta as its host even though the parasitoid frequently escapes encapsulation by host hemocytes. This report is the first description of post-emergence molting in larval hosts of endoparasitic Hymenoptera. Parasites diseases and disorders; Parasites; Insect parasites; Hosts; Insect hosts; Host parasite relationships Manduca sexta (Sphingidae); Hymenopteran parasites; Apanteles smerinthi; First record of host moult after parasite emergence; Reaction of host to parasite; Incomplete arrest of host larval development by hymenopteran parasite Apanteles smerinthi (Braconidae); Lepidopteran hosts; Manduca sexta; Cuticle secretion on parasites, incomplete arrest of host development; Reaction of host to parasite; Incomplete arrest of lepidopteran host developments