Postnatal development of vocalizations and audiograms was investigated in 19 juvenile Carollia perspicillata from the day of birth (age 0 days) to 7 weeks. In both juvenile isolation calls and adult social calls the 1st harmonic (fundamental) contained the most energy. Two types of isolation calls could be separated in young bats: “single notes” with 1 pulse and “double notes” with 2 pulses. Young bats 1st emitted echolocation pulses when they started to fly at age 17–22 days. Duration of these pulses was on average 1.8 ± 0.6 ms (mean ± SD; n = 11) and decreased during development to an adult-like duration of 0.9 ± 0.1 ms (n = 15) at 7 weeks. Frequency ranges of harmonics of the calls did not change significantly with age, but energy distribution of the harmonics was altered. Higher harmonics became successively emphasized. Newborn C. perspicillata hears 68% of the adult frequency range. During postnatal development the auditory range expanded upward. The upper limit of hearing, based on characteristic frequencies of multineuron clusters within the auditory midbrain (inferior colliculus), shifted from 77 kHz at birth to 110 kHz at day 50. Neural thresholds decreased most for frequencies of 60–110 kHz, which are most important for echolocation. However, from birth on, bats were most sensitive to low frequencies of 10–30 kHz.