In mammals, the size and number of spiral ganglion cells can vary significantly along the length of the cochlea. At present, it is unclear how these topologic differences in spiral ganglion cell morphology and density emerge during development. We addressed this issue by quantifying developmental changes in the number, density, and size of auditory ganglion cells within the cochlea of Mongolian gerbils throughout the first 3 weeks of postnatal life. In each cochlea, cells were measured at five standardized locations along the length of the spiral ganglion, as determined from serial reconstruction of Rosenthal's canal. During the first postnatal week, the total number of gerbil spiral ganglion cells decreased significantly by 27%, without further change thereafter. This brief period of neuronal cell death coincides with a major remodeling in the afferent neural projections to gerbil auditory hair cells (Echteler  Proc. Natl. Acad. Sci. USA 89:6324–6327). The resulting reduction in neuronal density varied with location, being most prominent within the upper basal and lower middle turns of the cochlea. These same regions contained the smallest auditory ganglion cells found within the gerbil ear and exhibited the least amount of developmental expansion in the circumference of Rosenthal's canal. These results suggest the possibility that regional differences in auditory neuron size and number might be influenced by local extrinsic factors, such as the availability of canal space. J. Comp. Neurol. 425:436–446, 2000. © 2000 Wiley-Liss, Inc.