Knowledge of trophic ecology is fundamental for understanding feeding strategies and niche dynamics, which can provide information on the vulnerability and conservation status of some poorly known species, such as Liolaemus monticola, a rock-dwelling lizard endemic to the central Andes of Chile. We studied its trophic ecology, analysing its diet and the availability of prey during three activity seasons (summer, autumn and spring). The results show that L. monticola is insectivorous, preying on epigean arthropods. The most frequent prey is formicid ants, on which it feeds in a greater proportion than that at which they occur in the environment. The annual diet is less variable than the seasonal fluctuations in the abundance of prey in the environment. We conclude that L. monticola is a trophic specialist with a preference for the ants Camponotus morosus, Araucomyrmex hypocritus and Brachymyrmex giardii. This diet presupposes an equilibrium between the relative toxicity of the ants, the energy cost of dealing with the prey and detoxification.