The development of social structures of the vampire bat (Desmodus rotundus) was studied in small groups in a captive colony over a period of 30 months. The membership of individuals in the main group and the development of group formation were evaluated by the coherence index. The main group of the vampire colony consisted of several adult females, their infants and one harem male. All members of the main group showed relatively high coherence indices, while the coherence index of adult males, except the harem male, was 0. The females and young in the main group formed a very stable group in comparison to the adult males. The young males left the main group at 20–27 months of age and formed temporary bachelor groups. The harem males changed during the study in the period from 1 to 20 months. They exhibited territorial behaviour and made vigorous attempts to approach the main group within the bounds of their roosting areas, whilst also keeping out intruders.