In the present study, the variations in the nutritional physiology and midgut ultrastructure of fourth-instar larvae of the Egyptian cotton leafworm, Spodoptera littoralis, treated with the essential oils of garlic (Allium sativum) and lemon (Citrus limon), were compared to those of control larvae. The food consumption index, relative growth rate and approximate digestibility of the treated larvae were significantly decreased. In contrast, the efficiency of conversion of both ingested and digested food to body mass was significantly increased. The midgut ultrastructural variations included vacuolisation of the columnar cells, clumping of the nuclear chromatin, disorder of microvilli, loss of cytoplasmic projections of the goblet cells, disorganisation of mitochondria and appearance of residual bodies. It seems likely that mitochondria are the target site of garlic and lemon essential oils. The overall results suggest that the essential oils of garlic and lemon under investigation interfere with the nutritional physiology and ultrastructure of the midgut of S. littoralis larvae and have potential for their control. Future studies will be undertaken to evaluate the efficacy of garlic and lemon oils under field conditions.