Unmyelinated (C) fibers are the most numerous sensory elements of mammalian peripheral nerve and comprise many of those responsible for initiating pain and temperature reactions; however, direct evidence has been lacking as to where and how these fibers terminate in the central nervous system. A plant lectin (Phaseolus vulgaris leukoagglutinin) was applied intracellularly by iontophoresis as an immunocytochemical marker. This permitted visualization of the central terminations of cutaneous C sensory fibers that had been identified by the nature of stimuli that excited them. The central branch of C-fiber units arborized and terminated mainly in the superficial layers of the spinal dorsal horn in defined patterns that related to their functional attributes. Thus, the superficial dorsal horn seems to act as a processing station for signals from fine sensory fibers.