The perception of sound is based on signal processing by a bank of frequency-selective auditory filters, the so-called critical bands(1-3). 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 neighboring laminae and may provide a spatial framework for the generation of critical bands and for signal processing within(4) and across’ frequency bands for the analysis of sound.