Roosting associations of Tylonycteris pachypus and T. robustula were studied in West Malaysia. Both species roosted within the internodes of one species of bamboo, but were only once found together at roost. Roosting groups, which formed before entry in the morning, were not limited in size by the dimensions of the roost site. In both species the sexes exhibited different roosting behaviour, the males tending to be solitary and the females gregarious. Seasonal variations in associations occurred correlated with the reproductive cycle. Banding demonstrated that the populations were highly mobile, that the bats frequently changed roost sites, and that the roosting associations were of an ephemeral nature.