The insecticidal activity of the leaf (ASAL) and bulb (ASAII) agglutinins from Allium sativum L. (garlic) against the cotton leafworm, Spodoptera littoralis Boisd. (Lepidoptera: Noctuidae) was studied using transgenic tobacco plants expressing the lectins under the control of the constitutive CaMV35S promoter. PCR analysis confirmed that the garlic lectin genes were integrated into the plant genome. Western blots and semi-quantitative agglutination assays revealed lectin expression at various levels in the transgenic lines. Biochemical analyses indicated that the recombinant ASAL and ASAII are indistinguishable from the native garlic lectins. Insect bioassays using detached leaves from transgenic tobacco plants demonstrated that the ectopically expressed ASAL and ASAII significantly (P < 0.05) reduced the weight gain of 4th instar larvae of S. littoralis. Further on, the lectins retarded the development of the larvae and their metamorphosis, and were detrimental to the pupal stage resulting in weight reduction and lethal abnormalities. Total mortality was scored with ASAL compared to 60% mortality with ASAII. These findings suggest that garlic lectins are suitable candidate insect resistance proteins for the control of S. littoralis through a transgenic approach.