The entire unmyelinated portions of 104 radial afferent fibers were reconstructed from serial ultrathin sections through the organ of Corti. Each radial fiber was unbranched and formed a single synapse with one inner hair cell. Fibers innervating the side of the hair cell facing the pillar cells were, in general, larger in diameter and richer in mitochondria than fibers innervating the side of the hair cell facing the modiolus. Some of the thin, mitochondrion-poor fibers had a more complex synaptic junction with the inner hair cell. A fiber classification scheme is suggested which may correlate with the classification of single auditory nerve units according to threshold and rate of spontaneous discharge .