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Warming climates are facilitating the range expansion of many taxa to habitats that were formerly thermally inhospitable, including to higher latitudes and elevations. The potential for such colonization, however, varies widely among taxa. Because environmental factors may interact to affect colonization potential, an understanding of underlying physiological and behavioral mechanisms is necessary to predict how species will respond to potentially suitable habitats. For example, temperature and oxygen availability will interact to shape physiological and performance traits. Our model species, the wall lizard, Podarcis muralis, is a widely distributed ectotherm that continues to expand its range in Europe despite being limited by cold temperatures at high elevations and latitudes. To test the potential for organisms to expand to warming high-altitude environments, we conducted a transplant experiment to quantify the within-individual effects of high-altitude hypoxia on physiological and performance traits. Transplanted lizards maintained individual differences in physiological traits related to oxygen capacity and metabolism (hemoglobin concentration, hematocrit, and peak postexhaustion metabolic rate), as well as performance traits tied to fitness (sprint speed and running endurance). Although lizards altered blood biochemistry to increase oxygen-carrying capacity, their performance was reduced at high altitude. Furthermore, lizards at high altitude suffered a rapid loss of body condition over the 6-wk experiment, suggesting an energetic cost to hypoxia. Taken together, this demonstrates a limited potential for within-individual plasticity to facilitate colonization of novel high-altitude environments.