Abstract: We measured δ 15N values and inferred the trophic positions of 151 ground ant species from four types of rain forests (alluvial, limestone, dipterocarp forest, and Kerangas) in Gunung Mulu National Park, in Sarawak, Malaysia. Four hypotheses were tested: 1) Ground-foraging ants occur in all trophic levels; 2) ant subfamilies differ in their trophic status; 3) δ 15N values differ among species within genera and among genera within subfamilies; and 4) ant assemblages in different forest types differ in their trophic structure. Base-line corrected mean δ 15N values for different ant species ranged from -0.67‰ to 10.56‰ thus confirming that forest ants occupy a variety of trophic levels. Based on stable isotopes we distinguished three major trophic groups: a) species mostly feeding on hemipteran exudates and other plant-derived food resources; b) omnivorous species with mixed diet of plant and animal prey; and c) truly predacious species, including arthropod specialists. Ant subfamilies differed significantly in their trophic positions, as did many ant genera within subfamilies and ant species within ant genera. Several ant species exhibited dietary flexibility and differed significantly in trophic positions across forest types.