Although defence against herbivores is often argued to be the main action of plant secondary metabolites (PSMs), very few examples have demonstrated that intraspecific variation in PSM concentrations influences foraging by wild vertebrate herbivores. Experiments with captive animals often indicate that PSM concentrations influence how much herbivores eat from individual plants, but these experiments do not replicate the subtle trade-offs in diet selection faced by wild animals, which must avoid predators and extremes of weather, interact with conspecifics, and achieve a balanced, nutritious diet, while avoiding intoxication by PSMs. We characterized the foliar chemistry of every tree from two Eucalyptus species available to a population of koalas (Phascolarctos cinereus) and considered rates of tree visitation over a ten-year period. We show that visitation rate was most strongly influenced by tree size, but that koalas also visited trees less frequently if the foliage contained either high concentrations of deterrent PSMs known as formylated phloroglucinol compounds, or low concentrations of nitrogen. Consequently, plant chemistry restricts the use of trees by this herbivore, and thus limits the food available to koalas and potentially influences koala populations.