A quantitatively accurate map of the tonotopic organization of the anteroventral cochlear nucleus (AVCN) was derived from single unit recordings. Histologically localized single unit recordings from many animals were mapped onto a computerized atlas of the cochlear nucleus, and surfaces of constant characteristic frequency (CF) estimated with the aid of computer graphics. In anterior AVCN the surfaces of constant CF were found to be parallel planes, whereas in posterior AVCN they progressively deviated from this simple description. A further complication was noted in the most posterior portion of the AVCN where units with very different CF was found in close proximity. Comparison of the tonotopic map with descriptions of cellular organization shows conclusively that different CF ranges are dominant in the various cytoarchitectonic regions of the AVCN.