This study reports on evidence for successful mating of Myotis daubentonii at summer sites established by paternity analysis and behavioural observations. Parentage in individuals with high site fidelity was verified. Associations of parents in the same roosts before September and repeated fatherhood of a single resident male support the hypothesis of regular mating activities at summer sites. Based on the assumption that successful mating of many European bat species occurs at summer sites after the young are fledged, anthropogenic habitat changes should steer clear of the seasonal activity period to prevent a reduced reproduction rate of local bat populations.