none The interactive effects of dietary nicotine (0.1%) and parasitism by Cotesia congregata (Say) (Braconidae) on post-ingestive utilization of food by the tobacco hornworm, Manduca sexta (L.) (Sphingidae) were studied in the laboratory. The rates of growth and consumption and the food utilization indices of third and fourth instar hornworms were not significantly influenced by the interactive effect of dietary nicotine and parasitism. However, parasitism reduced the growth rate of fifth instar hornworms, resulting from a reduction in their consumption rate. Efficiency of digestion of ingested food was enhanced in parasitized larvae, but not when they ingested diets containing nicotine. Ingestion of dietary nicotine reduced the efficiency of conversion of digested and ingested food of unparasitized hornworms as reflected ECD and ECI values, but these indices were larger than in parasitized hosts. Results suggest that when hornworm larvae are parasitized after ecdysis to the third instar, C. congregata does not influence feeding by its larval host until the host reaches its terminal instar. Nicotine, within host tissues, 'over-rides' any effect caused by parasitism. It is probable that any change observed may not result from a direct effect on the assimilation and conversion processes of the hornworm larva but due to parasitoids' exposure to nicotine in the hornworm's hemolymph.