Neurons in the central nucleus of the auditory midbrain inferior colliculus divide into four classes according to the shapes of their receptive fields. Neurons of two of these classes - sharply tuned, inhibition-dominated neurons of class II, and broadly tuned neurons of class III - show systematic gradients in their abundance on isofrequency contours. Sharp tuning is most prevalent in the center, broad tuning in the periphery of the ICC. This new map of tuning-curve shape adds to the six previously described maps of neural response properties on isofrequency contours of the ICC and stresses the fact that very different sensitivities and selectivities to sound properties are combined in local clusters of collicular neurons.