Summary: Big brown bat (Eptesicus fuscus) pups are born naked, blind, and with closed external ear canals that open asymmetrically. It remains unclear whether this developmental asymmetry influences hearing. To address this, we visually examined the ears of newborn bat pups (n = 34; 17 female) and found a difference in the days between when both ears opened (1 ± 0.5 days; t35 = –8.72, P < 0.001, 95% CI [0.92, 1.5]). Interestingly, in 72% of pups the left ear opened first. After both ears opened, we recorded auditory brainstem responses (ABRs) in 12 pups (6 male) by placing subdermal needle near the base of the auditory bullae, repeatedly presenting broadband acoustic clicks (bandwidth = 84 kHz; duration = 0.1 ms; n = 512 repetitions), and averaging the evoked neural response. The resultant averaged waveform represents the summed activity of neurons from different nuclei along the ascending auditory brainstem pathway. We compared monaurally- and binaurally-evoked hearing thresholds recorded from pups across the first month of life and at 3-months of age, and in adult female bats (n = 8). In pups less than 1 month old, monaural thresholds were lower in the ear that opened first compared to the opposite ear; however there was no longer a difference by 3-months of age. Binaurally-evoked thresholds tended to be lower (i.e. more sensitive) than monaurally-evoked thresholds for juvenile and adult bats. Our data suggest the initial asymmetry may help pups localize sounds until central circuits mature.