Within the broad continuum of polygyny, the mating systems of harem-forming bats exhibit a high degree of interspecific diversity and intraspecific variation. Here, I review the social structure and mating systems of 4 nominal species in the Old World fruit bat genus Cynopterus, and explore the relationship between roosting ecology, the size and stability of female groups, and variation in degree of polygyny. Intraspecific comparisons for C. brachyotis Sunda and C. sphinx suggest that the distribution and availability of suitable roost sites has a strong influence on the dispersion of females among males. Examination of paternity data for C. sphinx demonstrates a direct relationship between the environmental potential for polygyny and variance in male reproductive success: a 2-fold increase in mean harem size due to reduction in roost availability produces a 2-fold increase in reproductive skew among males. However, in C. brachyotis Sunda, C. sphinx, and C. horsfieldii, low fidelity of females to roosts or males is consistent, regardless of roost dispersion or permanence. In contrast, female C. brachyotis Forest exhibit high short-term fidelity to males, despite low roost fidelity. It is proposed that female lability is influenced by the distribution of potential mates and the benefits of multiple mating and, in C. brachyotis Sunda and C. sphinx, is likely to promote both behavioral and cryptic female choice, and male sperm competition.