Obtaining adequate levels of dietary protein is essential for the physiology of consumers. This presents potential problems for frugivorous birds because fruit is generally low in protein rendering it nutritionally inadequate and potentially explaining the rarity of exclusive frugivory in birds. We addressed this issue by determining the isotope composition ( 15N/14N) in the whole blood of two mistletoe consumers, that is, painted honeyeater (Grantiella picta, Meliphagidae) and mistletoebird (Dicaeum hirundinaceum, Dicaeidae) during the grey mistletoe (Amyema quandang, Loranthaceae) fruiting peak in a semi-arid woodland, NSW, Australia. Grey mistletoe fruit pulp and arthropods were isotopically distinct (mean δ15N fruit 4.4‰ vs. arthropods 7.1‰), thus readily discriminated using the stable isotope approach. Painted honeyeaters and mistletoebirds formed a single group based on their mean δ15N values and, on average, assimilated approximately half of their nitrogen from mistletoe fruit although individual variation was high. The importance of nitrogen derived from mistletoe fruit did not track its abundance in the environment, suggesting that at least during peak fruiting, this resource is not limiting at this site. Researchers should account for intraspecific variation and take a cautious approach when reconstructing diets using stable isotopes by incorporating individual-based analyses rather than presenting mean values alone. This is the first study to use the isotope approach to investigate the dietary relationship of mistletoe frugivores and mistletoe fruit and has implications for our understanding of the nutritional ecology of frugivores and its functional relationship to ecosystem processes such as seed dispersal. © 2009 Ecological Society of Australia.