We studied the population dynamics of the lesser long-nosed bat Leptonycteris curasoae in a cave in central Mexico for a two-year time. The population had substantial seasonal changes in size and composition during this time. Colony size was largest from February to July (22,000–27,000 adults) and contained equal numbers of males and females. In June and July, males had enlarged testes and presumably mated with females. In August, the population size began to decline and was mostly composed of pregnant females. Between September and December, the colony was composed exclusively of pregnant and lactating females and their young, and population size decreased to about one-third of the observed maximum size. In January, adult males returned to the roost and the population size increased. We also observed an increase in body mass and fat accumulation in both sexes, apparently related to reproductive activity. This is the first report of the continuous presence of a substantial female population of L. curasoae throughout the year in a single roost in Mexican tropics, indicating that some populations of L. curasoae in central Mexico complete their life cycle without having to migrate.