The changes of the number of new-born young were studied in Myotis macrodactylus among mixed bat colonies on every June from 1996 to 2006 at the Yashima caves in Takamatsu city, Kagawa prefecture, Japan. The identification for the species and the counting the number of the new-born young were carried out by enlarged pictures photographed in the caves. The new-born young were observed every year. The number of the new-born young was estimated as 255 on average per year. It was considered that the new-born young show nearly constant in number during the years investigated.