Social organization of a captive group of noctule bats, N. noctula, was studied by frequent censuses of bats at roost sites during one summer season. Numerous nonrandom associations as measured by information indices of similarity (Kullback 1959; Semkin 1972; Zakharov et al. 1987) were found between related and unrelated individuals. The social group structure revealed by multidimensional scaling analysis was affected first of all by the history of group formation, and also by bat age and sex. Spatial bonds were in general stronger between related than unrelated bats. However, nearly all closest associations observed were formed by at best distantly related bats; family membership influenced the socio-spatial positions of noctules far less than the above-mentioned variables did. It is suggested that emergence and persistence of nonrandom associations is mainly due to individual attainment of a certain level of comfort in company of familiar conspecifics whose behaviour is predictable, and thus reduction of aggression within a group. Other benefits of stable associations would be derived from this primary function.