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Seasonal variation in the consumption of cactus fruit by the bat Glossophaga longirostris was related to the availability of this resource in the arid pocket of Lagunillas in the Venezuelan Andes. When fruits were scarce, G. longirostris ate items not preferred in other seasons. The intra-specific flowering periods of the most common cactus species, Stenocereus griseus and Subpilocereus repandus, were synchronized but the number of flowers open per night was low. S. griseus and S. repandus had separate flowering and fruiting periods, possibly to reduce competition for dispersers and poliinators while providing food resources for bats throughout the year. G, longirostris is bimodally polyestrous with extended reproductive periods, probably as a response to the continual availability of cactus flowers and fruits. A strong interdependence between columnar cacti and G. longirostris was demonstrated. Sexual reproduction of the cacti would be greatly reduced without the bats, which in turn could not live in the arid study area without the food sources provided by these cacti.