We quantified the length of the forearm, the body mass and the length of the total gap of the fourth metacarpal-phalangeal joint of marked individuals of big-footed myotis bats (Myotis macrodactylus) from birth to flight in Dalazi Cave in Ji'an, Jilin Province, China. By using these data, we develop empirical growth curves, derive growth rates, establish age-predictive equations, and compare growth parameters based on three nonlinear growth models. Neonates of M. macrodactylus were born within a half-month period, with length of forearm averaging ca. 38.4% of the size of adult females and body mass averaging ca. 39.6% of their mother's body mass. Length of forearm increased linearly until 14 days after birth (growth rate = 1.20 mm/day), as did body mass until 11 days after birth (growth rate = 0.32 g/day). The length of the total gap of the fourth metacarpal-phalangeal joint decreased linearly from 13 days to 45 days of birth. Two most appropriate linear regression equations predicting the age of young bats were derived from forearm length ( ± SE) from 15.35 ± 0.96 mm to 33.24 ± 1.87 mm and the length of the epiphyseal gap from 13 to 45 days. These two equations make it possible to estimate the age of pups from 1 to 45 days of age in M. macrodactylus. Of the three nonlinear growth models (logistic, Gompertz, and von Bertalanffy), the logistic equation provides the best fit to the empirical curves for length of forearm and body mass.