1. Red wood ants are among the most numerous generalist predators and strongly affect the composition of arthropod communities in forest ecosystems. However, their trophic position remains poorly understood. Stable isotope analysis was applied to study the trophic position of Formica aquilonia and reveal seasonal changes in its trophic links with both myrmecophilous aphids and other invertebrates in a mixed forest of western Siberia. 2. The δ15N values of F. aquilonia exceeded those of herbivores and aphids by approximately 3.5‰. Despite obligate trophobiotic relationships with aphids, F. aquilonia occupied the trophic position of first-order predator. The higher content of 13C in the worker ants, compared with members of grazing food chains, was explained by their consumption of 13C-enriched aphid honeydew. 3. Myrmecophilous tree-dwelling aphids were enriched in 13C and 15N relative to grass-inhabiting species, and the honeydew of tree-dwelling aphids had higher δ13C values than those of the honeydew of grass-inhabiting aphids. 4. The decrease in δ13C values of the worker ants from spring and summer to autumn apparently reflected the transition from the collection of tree sap and feeding on the aphid honeydew from trees with high 13C content in the spring and early summer to a more diverse liquid diet in late summer, which included 13C-depleted honeydew of aphids from herbs. 5. The prevalence of the 15N-depleted aphid honeydew in the ants' diet in the second half of the summer is discussed as one possible explanation for the seasonal decline in δ15N values of the worker ants.