There is communication and social synchronization of the circadian rhythm in the flight activity of the microchiropteran cave-dwelling batHipposideros speoris. Thus captive bats surrounded by free-flying conspecifics synchronize their activity to the colony activity. The circadian rhythm of a solitary bat in a solitary cave freeruns. Even the rhythm of an ‘alien’ bat (Taphozous nudiventris kachhensis) held captive in the hipposiderid bat cave freeruns. But the rhythms of a closely-related species,Hipposideros fulvus partially entrain to social cues fromHipposideros speoris. Social synchronization of circadian rhythms in bats may be species-specific. This synchronization is abolished when continuous light of 10–20 lux is shone inside the natural cave.