Some cave-dwelling bat species such as Myotis myotis now also roost in buildings in parts of their range. I studied the effects of roost temperature and colony size on reproductive success of colonies of M. myotis located in buildings. Temperatures in attics fluctuated considerably and altered the thermal conditions at the roost during the breeding season. Inter-colony variation was recorded with respect to factors indicating reproductive success (number, growth, and mortality of juveniles until fledging). However, over-winter survival varied more between years than between colonies. Colony size did not influence development or mortality of juveniles. High average roost temperatures favoured development of juveniles but did not influence mortality. Unlike the situation in caves, large colony sizes are not advantageous in buildings