Abstract
Chromosome numbers have been determined from the bone marrow cells of the five vespertilionid bats. In three species of the genus Pipistrellus the diploid numbers are 2 n=34, 36 and 38. In two species of the genus Scotophilus the diploid number is 2 n=36. It is suggested that Robertsonian type of mechanism (centric fusion or fission) has played a major role in the karyotype evolution in the genus Pipistrellus.