Differences in trophic niches among carabid beetles (Coleoptera: Carabidae) co-occurring on the forest floors of warm temperate forests in central Japan were studied using carbon (delta(13)C) and nitrogen (delta(15)N) stable isotope analyses. Different carabid species showed similar delta(15)N values, which were higher than those of their possible invertebrate prey (herbivores and detritivores) collected from the litter layer, indicating that these species were consumers in the same trophic level. In contrast, delta(13)C values differed among carabid species, indicating interspecific differences in prey animals. The variation in the delta(13)C value was larger in summer than in autumn. In summer, delta(13)C values indicated that some carabids depended highly on either grazing (low delta(13)C values) or detrital sources (high delta(13)C values) within the food chain [Chlaenius posticalis Motschulsky and Haplochlaenius costiger (Chaudoir), respectively], although other species with intermediate delta(13)C values likely depended on both. The latter group of species comprised mostly two dominant genera (Carabus and Synuchus). Although congeners might have similar feeding habits, the stable isotope ratios indicated trophic niche differences between adults of different species and between adults and larvae of the same genus.