ABSTRACTLowland Amazon is climatically one of the least seasonal regions on the planet, but little is known about how this is reflected in ecological seasonality. The central objective of this study was to determine whether seasonal fluctuations in the availability of fruit resources in Neotropical forests are sufficiently marked to affect the ecology and physiology of frugivorous bats. Seasonal variations in the overall bat abundance and in the captures, body condition, and reproductive activity of the two most abundant species,Carollia perspicillataandArtibeus planirostris, were studied within a region of central Brazilian Amazonia dominated by a mosaic of nonflooded (terra firme) and seasonally flooded forests (várzeaandigapó). Concurrent seasonal changes in fruit availability were measured. The abundance of fruits was markedly seasonal, with far fewer resources available during the low‐water season. There was a positive correlation between fruit and bat abundance. Overall, bats did not increase the consumption of arthropods during the period of fruit shortage. InA. planirostris, the body condition declined when fruits were scarcer. In bothC. perspicillataandA. planirostris, foraging and reproductive activity were positively correlated with fruit availability. Consequently, the results suggest that resource seasonality is sufficiently marked to affect frugivorous bats and force them to make important eco‐physiological adjustments.