Short-tailed fruit bats typically breed twice a year, fitting a seasonal bimodal polyestry pattern in many ecosystems along their distributional range. For two species, Carollia perspicillata and C. benkeithi, reproductive timing was studied on a monthly basis and related to environmental seasonality (climatic factors and fruit resource availability) in a Peruvian premontane forest with a secondary succession plant composition. Sex, age and reproductive condition were assessed through morphological inspection. Plant species, whose fruits were known to be consumed by these bats, were scored for fruit resource availability. For C. perspicillata, lactation followed a bimodal pattern with peaks at the onset (September–October) and the middle of the rainy season (February–March). The proportion of lactating females was positively correlated with rainfall, temperature and fruit resource availability. In contrast, reproductive males, as indicated by descended testes, occurred during the dry season and its proportion was negatively correlated with rainfall. For C. benkeithi, records of reproductive females suggest that births also occur during the rainy season. This study supports the previously suggested synchronization of births with higher fruit resource availability for Carollia species, and contributes with information about the geographic variation of their reproductive phenology.