Tropical rainforests are among the most complex and diverse ecosystems, composed of a mosaic of shady understory under the closed canopy and tree-fall gaps of varying sizes and age. The light reaching the forest floor favors the recruitment of fast growing plant species and provide food resources for other animal species including butterflies. The Nymphalidae are the most species rich butterfly family in the tropics, and are ideal bioindicators. We investigated the effect of the tree-fall gaps on the assemblages of fruit feeding Nymphalidae. We used fruit-bait traps in 15 tree-fall gaps from 100 to 1000 m2 and 15 in undisturbed understory, from July until November, in a lowland tropical rainforest in southeastern Peru. We found distinct differences in butterfly assemblages between tree-fall gaps and understory, with a higher number of species in gaps, associated with a higher level light. We identified several species mostly found in one of the habitats, and generalist species. The heterogeneity was large within the same site both in gaps and in the understory. The difference between butterfly assemblages increased with gap size. Butterfly species were mainly associated with the absence of vines in the gaps, and found in large and light gaps. We distinguished several species according to their preferences for the vegetation structure, light level and size of gaps. We concluded that one example that maintains the biodiversity in the tropical rainforest is the formation of tree fall gaps of different sizes resulting in different species assemblages.