The perception of sound is based on signal processing by a bank of frequency-selective auditory filters, the so-called critical bands. Here we investigate how the internal frequency organization of the main auditory midbrain station, the central nucleus of the inferior colliculus (ICC), might contribute to the generation of the critical-band behaviour of its neurons. We find a unique spatial arrangement of the frequency distribution in the ICC that correlates with psychophysical critical-band characteristics. Systematic frequency discontinuities along the main tonotopic axis, in combination with a smooth frequency gradient orthogonal to the main tonotopic organization of cat ICC, reflect a layering of the frequency organization paralleling its anatomical laminae. This layered frequency organization is characterized by constant frequency ratios of corresponding locations on neighbouring laminae and may provide a spatial framework for the generation of critical bands and for signal processing within and across frequency bands for the analysis of sound.