A mating colony of Nyctalus leisleri has been monitored since 1994 through the periodic checking of 105 bat boxes set in a beech forest on the Apennines (Tuscany, central Italy). Both a general and a gender-specific trend was obtained from the monthly count of individuals. The general trend showed remarkable numerical fluctuations over the years, which were particularly evident in the number of females. The overall population showed a slight positive trend, which was significant for males. The abruptly fluctuating number of females observed throughout the years was worthy of notice. Such variations may find an explanation when more information on the continental migration patterns of the individuals recorded in this area become available.