The role of auditory experience in the ontogeny of perceptual coding was investigated in hatching chickens. In Experiment 1, auditory frequency generalization gradients were determined by using a habituation-generalization paradigm and an orienting response. One-day and 3-4 day chicks were habituated to 800-Hz tones and then tested at five frequencies ranging from 800 to 1,000 Hz. One-day chicks displayed reliably flatter generalization gradients than 3-4-day chicks. In Experiment 2 an auditory deprivation method is described in which an ear impression compound was injected into the external ear. This method (a) is fully calibrated and provides approximately 40 dB of attenuation across a .125- to 4.0-kHz range, (b) appears to be fully reversible, and (c) is applicable prior to normal auditory experience as well as at subsequent stages. In Experiment 3, this method was used to investigate the effects of auditory deprivation on the "perceptual sharpening" described in Experimental 1. Chicks were deprived from Embryonic Day 18 1/2 to 3-4 days posthatch, at which time normal, deprived, and sham-operated chicks were tested as in Experiment 1. The generalization gradients of the deprived chicks were significantly flatter than those of the 3-4 day normal and sham-operated chicks and were similar to those of 1-day chicks. This effect cannot be attributed to (a) operative procedures, (b) changes in auditory thresholds, or (c) changes in response rate with age or condition. Thus the perceptual sharpening normally occurring between 1 and 3-4 days posthatch fails to occur during that time in the absence of a normal acoustic environment.