In southern Arizona, heat stress plays a significant role in the interaction between the pipevine swallowtail butterfly Battus philenor and its Aristolochia watsoni host. Heat stress for the butterfly in the desert environment is compounded by the highly recumbent growth form of the plant which exposes eggs, caterpillars and ovipositing females to a thermal cline with temperature maximums at the soil surface commonly in excess of 65^oC. Field data indicates that degree of stress varies depending on habitat, with plants in open washes being associated with a higher degree of heat stress for both caterpillars and adult females. Caterpillars suffer in the wash habitat from two challenges: first, more recumbent growth form in wash plants generates a more extreme thermal cline than found in the grass habitat and second, the scarcity of nonhost vegetation causes caterpillars experience difficulty in finding thermal refuges off host in the midday heat. Presumably in part as a consequence of the adverse effects of high temperatures on juvenile fitness, females strongly avoid oviposition on wash plants. Laboratory data are presented that such discrimination may be due partly to a direct effect of high temperatures on female oviposition and partly to an indirect effect mediated through heat-induced changes in the host plant. Finally, evidence is presented that ovipositing females in laboratory assays readily learn to avoid microhabitat cues associated with exposure to heat stress under wash habitat conditions. The significance of this work is discussed in light of increases in temperatures predicted by climate change models.