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Estimates of postnatal growth rates and age based on data collected from free-ranging little brown myotis (Myotis lucifugus) captured sequentially at the same colony in the same year were used to compare longitudinal (mark–recapture) and cross-sectional (grab) sampling methods. Analyses of these data indicate that cross-sectional sampling significantly underestimates growth rates for length of forearm, body mass, and length of epiphyseal gap. Equations based on cross-sectional samples significantly overestimated ages of young, as compared with equations based on the longitudinal method. These results support the hypothesis that cross-sectional sampling is unreliable for deriving postnatal growth curves for free-ranging bats and emphasize the importance of using longitudinal data to derive growth rates and estimates of age during the postnatal period.