We have analyzed the effects of Toxoneuron nigriceps parasitization on the midgut development of its host Heliothis virescens. In parasitized H. virescens larvae, the midgut epithelium undergoes a complete replacement, which is qualitatively not different to that observed in synchronous unparasitized larvae, with similar temporal profiles of cell death and metabolic activity. However, the whole gut replacement process is significantly delayed in parasitized larvae, with complete differentiation of the new gut epithelium being observed 4 days later than in unparasitized controls. The administration of juvenile hormone before commitment and of 20-hydroxyecdysone (20E) after commitment delays and fosters, respectively, the replacement process of the midgut epithelium; moreover, the injection of 20E into developmentally arrested and 20E-deficient host last-instar larvae parasitized by T. nigriceps immediately triggers regular gut development. These hormone-based experiments suggest that endocrine alterations in the larval host, induced by T. nigriceps parasitism, are responsible for the temporal alterations in the gut replacement process. The role of this parasitoid-induced developmental change in the host regulation process is discussed.