The maturation of the auditory nerve's ability to encode temporal information in an acoustic signal was studied in young kittens, 7 to 23 days old, and in adult cats by measuring the degree to which auditory nerve fiber responses are synchronized (phase locked) to low frequency tones. The major findings include the following. In 7- to 10-day-old kittens thresholds are high (around 100 dB), and secure phase locking is observed only at frequencies below about 600 Hz. The upper frequency limit for phase locking in the adult, around 4 kHz, is reached gradually toward the end of the third postnatal week, a time when thresholds also reach their adult levels. The time course of development of the phase-locked response is similar for fibers with different characteristic frequencies (CFs). At all ages studied, the maximal phase synchrony at any given low frequency is generally obtained at stimulus intensities less than 20 dB above the rate threshold and some 10 to 30 dB below the saturation firing level of the fiber. Time delays, estimated from phase-versus-frequency plots, were similar across ages, although they tended to be longer and more variable at lower CFs during the first 2 postnatal weeks. Comparing the phase-locked response of auditory nerve fibers with that of the anteroventral cochlear nucleus neurons from a previous study suggested that the upper frequency limit for phase synchrony was reached later in the nucleus than in the nerve.