The population of hammer-headed bats, Hypsignathus monstrosus H. Allen, 1861, in the District of Plateau in Abidjan has been studied from August 2003 to July 2004. Vocalization recognition and roost counts of bats males have been used in order to identify mating calls sites and to determine the variations of population size. Results show that the hammer-headed bats have demonstrated a high preference for reproduction site dominate by Terminalia catappa L. (Combretaceae). The population size varied with the site, the tree species and season.