In order to ascertain the form in which newly synthesized membrane is moved by fast axonal transport, we examined the distribution of label along an axon of the identified giant serotoninergic neuron (GCN) in the cerebral ganglion of Aplysia californica. Membrane glycoproteins were labeled by intrasomatic injection of 3H-fucose, and segments of GCN's axon in the posterior lip nerve containing the transported organelles were examined at 1, 5, 15, and 24 hours by quantitative electron-microscopic autoradiography. To show that membrane which is rapidly transported is contained only in discrete organelles rather than in continuous sheets of axoplasmic reticulum, we systematically varied conditions of fixation. We found that we could distinguish vesicles from axoplasmic reticulum most reliably in axons fixed with 2% formaldehyde and 2% glutaraldehyde in cacodylate buffer. At short times after intrasomatic injection of 3H-fucose, dense-cored vesicles and multivesicular tubules were the only axonal organelles labeled.