Scotinomys xerampelinus, the long-tailed singing mouse, is diurnal and insectivorous, and exhibits a complex and unique calling behavior that is audible to humans. Little is known about the social structure of this species. We used livetrapping and radiotracking to investigate the spatial organization of a wild Panamanian population of long-tailed singing mice. We observed exclusive space use among females but not among males. For both males and females, individual home ranges (85% minimum convex polygons) typically overlapped with >1 animal of the opposite sex. No significant differences in body mass, hind-foot length, or home-range size (area) were found between males and females. Most individuals in the population nested alone. Patterns of space use and sexual dimorphism are frequently used to infer species' mating systems. Our results, in particular the tendency for individual home ranges to overlap with multiple potential reproductive partners, are most consistent with a promiscuous mating system.