We studied the paternity in a colony of the harem-polygynous white-lined bat Saccopteryx bilineata by microsatellite typing and compared the data with group composition and stability. Although we recorded a high stability for harem groups, neither spatial proximity of males to harem females nor harem ownership allowed us to predict the paternity of the next year's harem offspring. Eight out of 28 juveniles were fathered by holders of the harem in which they were born, while the other 20 represent Extra-Harem-Young (EHY). 50% of EHY were fathered by males from outside the colony and 50% by other harem holders or peripheral males of the colony. On average, reproductive success of harem holders (1.2 offspring/year) was higher compared with peripheral males (0.4 offspring/year). Harem size seemed not to influence reproductive success of harem holders. Although maintaining of a territory seems to be costly for a harem male, his ability to control the females of his harem may be restricted; instead female Saccopteryx bilineata appear to have a high potential for female choice.