The long-fingered bat Myotis capaccinii (Bonaparte, 1837) is considered rare and is listed as vulnerable on the IUCN Red List of Threatened Species (2007). It remains one of the least studied bats in Europe. Protection of roosts is fundamental to bat conservation but, for it to be effective, knowledge about roosting ecology is required. We evaluated roosting habits, colony structure, and colony composition of M. capaccinii in the National Park of Dadia–Lefkimi–Soufli in Greece during 2002–2004 at 6 underground sites. We report results from regular capture of marked individuals and measures of roost microclimate. Individuals formed large maternity colonies in spring. Parturition began in April and by the end of May all captured females had given birth. The 1st volant young appeared in late June and almost all were weaned by the end of July. Summer colonies were sexually segregated; few males were present in the nursery roosts. Adult females began to disperse in August and the proportion of males increased through September and October. Young born in the year remained in the roosts after the females had left. M. capaccinii selected summer roosts with a wide range of temperatures, tolerating temperatures as low as 11.2°C, probably because of the thermoregulatory benefits of aggregating in single- and multispecies clusters. Few individuals remained in these sites during the winter. The majority of females appeared to reach sexual maturity in their 1st autumn and most males did so in their 2nd year. Body mass of males increased steadily from spring through to autumn, whereas mass of females only increased in the autumn before hibernation, except for the additional weight of the fetus during pregnancy, implying that reproduction imposes energetic constraints on females.