Abstract: Genetically modified maize with the insecticidal capacity of Bacillus thuringiensis(expressing Cry1Ab) represents almost 80% of the total maize sown in the study area (Lleida,Catalonia, NE Iberian Peninsula). Bt corn is highly effective for controlling maize borers but 2other non-target Lepidoptera, Mythimna unipuncta (Haworth) and Helicoverpa armigera(Hübner), cause occasional but severe damage to maize in the study area. Previous studies carriedout in the laboratory and the field on the effect of Bt maize on these 2 Lepidoptera demonstratedthat some larvae of the 2 species can survive and complete their development when feeding on Btmaize. In this study the suitability of a diet containing lyophilized maize leaves for trialsconducted to evaluate the susceptibility of H. armigera larvae to Bt maize varieties wasdemonstrated. Bt toxin produced a quite steady cumulative mortality in the different larval instarsof H. armigera. The Bt protein was well detected in the lyophilized Bt leaves and in the dietprepared with lyophilized leaves. In the H. armigera larvae the protein was detected in thecontent of the peritrophic membrane, in the midgut and in the hemolymph of the larvae fed on aBt diet. Once ingested, the majority of the toxin was degraded inside the peritrophic membraneand the growth rate of these larvae was lower than that of larvae fed on a non-Bt diet. These maybe some of the reasons for the low susceptibility of H. armigera larvae to the Bt toxin.