Triton X-100 insoluble neurofilament (NF) fractions were obtained from two parts of the stellate ganglion and the main giant axon. These were analyzed by one- and two-dimensional gradient polyacrylamide gel electrophoresis, cyclic assembly and disassembly, and electron microscopy. The NF fractions from the ganglion cell bodies (GCB) and from the part of the ganglion mainly consisting of axon initial segments (GIS) were of similar composition; neither contained detectable amounts of the 220 kda and high molecular weight (greater than 400 kda) NF subunits that were prominent in the axonal NF fraction. However, the GCB and GIS did contain large quantities of a set of 65 kda polypeptides that were minor constituents of the axonal NF fraction. The 65 kda-containing NF fraction from the ganglion could be cyclically disassembled and reassembled, but only under low salt conditions, in contrast to the high salt conditions used to cycle axonal NFs. A comparison of the peptide map of the 65 kda polypeptides with that of the 60 kda axonal NF subunit showed them to be different. These biochemical differences between the ganglionic and axonal NF fractions correlated with morphologic distinctions: ganglionic NFs were relatively smooth surfaced, whereas axonal NFs had long sidearms. Such observations support the hypothesis that the NF cytoskeleton of the neuron soma is different from that of the axon. Furthermore, the change from the somal form to the axonal form of NFs appears to occur in the region where the axon initial segment increases in diameter to become the axon proper.