Bats spend their day and winter time in roosting site such as tree, hollows, cave or human made structure to hibernate or retreat. Roosting preferences can be different according to species. While some of them tend to be solitary, others in colony which is consist of individuals from same or different bat species. In this study, it was investigated to roosting behavior of different bat species in 15 caves of Middle Sakarya Region, Turkey. In 2012-2013 period, each cave was visited three times for spring, summer and winter seasons. Totally 10 bat species were detected in the caves. All of the species except Myotis emarginatus were roosted not only singly but also in groups. M. emarginatus was found only as a solitary. It was determined 70 groups for 9 species. Number of individuals in the groups varied between 2 and 910. Ten of the all groups were formed by mixed species. The percentage of the mixed groups according to colonies in spring, summer and winter were 29,5%, 25% and 0% respectively. Roosts were shared with minimum 2 maximum 3 species in mixed groups. M. blythii shared its roosting sitewith M. myotis and M. capaccini separately. Also, Rhinolophus blasii grouped with R. mehelyi. On the other hand, M. myotis, M. blythii ve Miniopterus schreibersii were detected in the same group.None of the individuals of the M. emarginatus, R. hipposideros, R. ferrimequinum and R. euryale tended to colony with different species. All groups were formed by conspesific in winter season, as well. Studies on the group formation of bat species confirm our finding that individuals usually do not prefer to mixed colony to hibernation. On the other hand, tendency to aggregate with different species in spring and summer time can be due to the same habitat requirements such as food. This study was supported by a scientific research grant (1205F087) from Anadolu University Research Fund.