Plants often face chronic seed loss from predispersal seed predation by insects.Although many studies have documented the rates of seed loss for single species in different communities, it is unclear how rates of predispersal seed predation vary among co-occurring species within the same community. If interspecific asymmetries in seed loss are great, this common interaction could have important implications for coexistence. Species traits, such as seed size or seed nitrogen and carbon, might correlate with interspecific variation in predispersal seed predation among co-occurring grassland forb species. We collected infructescences from 13 co-occurring forb species from each of four western Montana grasslands over 2 years. We quantified the magnitude of seed loss due to predispersal seed predation by insects. The average level of seed loss was 15.8%. Larger seeded species suffered significantly higher levels of predispersal seed predation than smaller seeded species, and seed size predicted seed loss more than seed nitrogen or carbon. Although large-seeded species often have greater proportional recruitment and early survival than small-seeded species, our study suggests that these advantages are partially counterbalanced by greater predispersal seed loss for larger versus smaller seeded species. Asymmetries in predispersal seed predation may importantly affect coexistence among these species.