Postnatal changes in body mass and flight-related morphological features were monitored by mark-recapture method in a breeding colony of Geoffroy's bat Myotis emarginatus in Kerend cave, western Iran. At birth, neonates' average and standard deviation of body mass, length of forearm, and length of epiphyseal gap were 2.57 ± 0.33 g, 17.37 ± 1.60 mm, and 2.65 ± 0.42 mm, respectively. Body mass and forearm length of pups increased monotonically until 20 days, when they achieved 72.37% and 91.70% of the values of postpartum females and thereafter growth rates decreased. Epiphyseal gap length increased to its maximum size at about 8–10 days after birth and subsequently decreased. Wing characteristics including wingspan, wing area, handwing length, handwing area, armwing length, and armwing area increased monotonically until the age of the first flight, after which growth rates significantly declined (all P < 0.05). Pups achieved sustained flight at 25–30 days of age. Wing loading of pups was 11.28 ± 1.27 Nm^-2 and decreased monotonically to 5.99 ± 0.68 Nm^-2, but aspect ratio remained fairly constant. Regression analysis indicates that handwing length and forearm length provide the most accurate estimate age of the young of My. emarginatus.