Analysis of stable isotope ratios is increasingly used to reconstruct diets in passerine birds, but studies of diet-tissue isotopic discrimination for this avian group are scarce. We determined 15N and 13C diet-tissue discrimination factors on whole blood in the red-throated ant tanager (Habia fuscicauda), an insectivorous-frugivorous passerine. Birds were fed an isotopically uniform, semi-synthetic diet of dog puppy dry food, soy protein isolate, wheat germ, and other ingredients, during 92 days. Average (± SD) diet-tissue discrimination was 2.6 ± 0.2‰ for N and 2.2 ± 0.1‰ for C. Nitrogen diet-tissue discrimination was similar to the values found previously in other passerines fed animal protein and it can probably be used to accurately reconstruct protein dietary origin in passerines feeding on animal protein (e.g., insects). In the case of C, diet reconstruction might be affected by metabolic routing of dietary nutrients. © Dt. Ornithologen-Gesellschaft e.V. 2007.