We collected data on the diet of eight species of insectivorous bats (Chiroptera, Vespertilionidae): big brown bats (Eptesicus fuscus), red bats (Lasiurus borealis), evening bats (Nycticeius humeralis), northern myotis (Myotis septentrionalis), little brown myotis (M. lucifugus), Indiana myotis (M. sodalis), southeastern myotis (M. austroriparius) and eastern pipistrelles (Perimyotis subflavus). Bats were mist netted during the summers of 1999 and 2000 at 41 forest sites throughout southern Illinois. We analyzed prey remains in fecal pellets of 305 individuals to assess diet similarity among species and relationships between bat body mass and prey diversity and hardness. Larger species included big brown bats and evening bats that ate primarily hard-bodied beetles (Coleoptera). These bats had the greatest dietary similarity index value compared with the other chiropterans in the community, and the highest hardness indices of prey consumed. Red bats, second only to E. fuscus in mean body mass, ate more soft bodied moths (Lepidoptera) and leaf hoppers (Homoptera) than beetles. Smaller bats, including three species of Myotis, consumed the greatest diversity of prey and were generally grouped together in diet similarity indices. Little brown myotis, northern myotis and Indiana myotis fed primarily on moths and beetles. Northern myotis and little brown myotis also fed extensively on spiders, suggesting significant gleaning behavior. Unlike other Myotis, the southeastern myotis had a low dietary diversity index and fed primarily on caddisflies (Trichoptera), as did eastern pipistrelles. Pipistrelles and myotines had the lowest hardness indices of prey consumed. Bats in southern Illinois exhibited landscape level (macroscale) feeding patterns consistent with the predicted relationship between body size and hardness of prey consumed, while at the local, site-specific level (microscale) they foraged with extensive overlap among similar-sized species, especially most Myotis. Regional differences in diets were minimal within the same assemblage of bat species in southern Indiana.