Body mass variations of Pipistrellus kuhlii and Eptesicus isabellinus were studied in the Bou Hedma National Park (central Tunisia) from June 2010 to June 2011. In both species, adult females were significantly heavier than males throughout the year. Seasonal variations were larger in adult females, body mass peaking during late pregnancy. After a secondary peak in September–October observed in both sexes in E. isabellinus, in males only in P. kuhlii, body mass reached a minimum value in winter when activity is low. Significant differences were also noticed between years in June for both species. Such pattern of seasonal variations of sexual size dimorphism is typical of hibernating vespertilionids.