The feeding behaviour of specialist butterflies may be affected by the mechanical and chemical characteristics of the tissues of their host-plants. Larvae of the butterfly, Battus polydamas archidamas feed only on Aristolochia chilensis, which contains aristolochic acids. We studied the oviposition pattern of adults and the foraging of larvae of B. polydamas archidamas over time in relation to variations in hardness of the substrate and concentration of aristolochic acids in different plant tissues. We further tested the effect of two artificial diets containing different concentrations of aristolochic acids on larval performance. B. polydamas archidamas oviposited mostly on young leaves and the larvae fed on this tissue until the second instar. Third instar larvae fed also on mature leaves and fourth and higher instars fed also on stems. Young leaves are softer and contain higher concentrations of aristolochic acids than mature leaves, and stems are both harder and contain a high concentration of aristolochic acids. Larvae reared on artificial diets containing a high concentration of aristolochic acids suffered less mortality and were heavier than those reared on a diet with a lower concentration of aristolochic acids, which suggests they are phagostimulatory. A strategy of host use regulated by aristolochic acid content and tissue hardness is discussed. PUBLICATION ABSTRACT