Classic theories on assembly rules and food-web structure are species-centered, so they largely ignore intraspecific variation. Intraspecific trophic variation, however, might be of key importance in understanding community organization. Here we study the variability of isotopic niche spaces of ground beetles and its consequences for the trophic structure of beetle assembly. Stable isotopes ratios (δ¹³C and δ¹⁵N) were measured in 1156 specimens of carabids belonging to 59 species inhabiting 20 island and two mainland sites of the Masuria Lakeland in northern Poland. Carabid species belonged to three different trophic guilds (named “phytophages”, “decomposer feeders” and “generalist predators”). However, this division is not sharp due to high intraspecific variability of isotopic signatures, which indicates the use of very different types of resources by conspecific individuals inhabiting different sites. As a consequence, most species studied did not differ significantly in the isotopic niche space. This high niche overlap corroborates the view that resource competition is not a major factor shaping the composition of ground beetles communities. Future studies should take into account the complex trophic structure of beetle assemblages and explore the intraspecific niche variability of ground beetles.