Patterns of growth and development provide information critical for documenting population demographics. Herein we document the reproductive biology for and provide equations that accurately predict the ages of 2 widely sympatric species of bats from central Indiana—the northern myotis (Myotis septentrionalis) and the little brown myotis (Myotis lucifugus). Young of M. septentrionalis were born synchronously (within a 6-day period) and within a narrow size range (mean length of forearm = 12.78 mm, 95% confidence interval [95% CI] = ± 0.52 mm; mass = 1.91 g, 95% CI = ± 0.11 g). Young of M. lucifugus were born during a 42-day period and within a narrow size range (mean length of forearm = 14.26 mm, 95% CI = ± 0.23 mm; mean body mass = 2.19 g, 95% CI = ± 0.08 g). Young of M. septentrionalis were born significantly smaller (t = 4.52, d.f. = 137, P < 0.001) and lighter (t = 2.57, d.f. = 137, P < 0.05) than were young of M. lucifugus. We developed species-specific equations that made it possible to estimate the age of both species of bat to within 1 day. These results indicate that similar-sized and closely related bats grow at comparable rates and provide the 1st detailed information about reproductive biology of M. septentrionalis.