To evaluate how the Cry1Ac Bacillus thuringiensis (Bt) toxin and the ratio of dietary protein to carbohydrate (P:C) independently and jointly affect Helicoverpa zea (Boddie) larval feeding performance and behavior, we conducted no-choice and binary choice experiments using chemically defined diets. We tested two related strains of this polyphagous pest: a strain selected for resistance to Cry1Ac in the laboratory (GA-R), and its field-derived parent strain (GA). In no-choice tests, feeding performance in GA and GA-R was superior on a non-Cry1Ac 80P:20C diet compared to a non-Cry1Ac 35P:65C diet or an 80P:20C diet containing Cry1Ac. Based on consumption and position measurements in binary choice tests, larvae preferred the non-Cry1Ac 80P:20C diet over the other two diets mentioned above. However, the association between preference and performance was weaker when comparing the Cry1Ac 80P:20C diet versus a Cry1Ac 35P:65C diet, and when comparing the Cry1Ac 80P:20C diet versus the non-Cry1Ac 35P:65C diet. In all choice situations, consumption preference occurred mainly by the percentage of larvae that fed almost entirely from one or the other diet, rather than from variation in the extent of diet mixing by individuals. Resistance to Cry1Ac affected the balance between toxin avoidance and nutrient intake: larvae from the more resistant GA-R strain consumed significantly more of the Cry1Ac 80P:20C diet when paired with non-Cry1Ac 35P:65C diet, while GA larvae consumed more of the latter, though not significantly. The results show that dietary P:C ratio, Cry1Ac, and resistance to Cry1Ac affected feeding behavior and performance of H. zea larvae.