In crayfish, the severed distal segment of single lateral giant axon (SLGA) often survives for at least 10 months after lesioning if this segme;t retains a septal region of apposition with an adjacent, intact SLGA. In control (unsevered) SLGAs, this septal usually contains gap junctions and 50-60 nm vesicles near the axolemma of both SLGAs. From 1-14 days after lesioning, the distal segment of a severed SLGA undergoes obvious ultrastructural changes in mitochondria and neurotubular organization compared to control SLGAs or to adjacent, intact SLGAs in the same animal. Gap junctions are very difficult to locate in severed SLGAs within 24 h after lesioning. From two weeks to ten months after lesioning, the surviving stumps of severed SLGAs often appear remarkably normal except that structures normally associated with the presence of gap junctions remain very difficult to find. These and other data suggest that SLGA distal segments receive trophic support from adjacent, intact SLGAs. The mechanism of this support probably could not be via diffusion across gap junctions between intact and severed SLGAs since gap junctions largely disappear after lesioning. However, trophic maintenance could occur via the exocytotic - pinocytotic action of 50-60 nm vesicles which are always present on both sides of the septum between an intact SLGA and a severed SLGA distal segment.