The dermal melanocytes in the lesions of nevus of Ito and Mongolian spot were shown to possess the extracellular sheath composed of both fine filaments and granules (2-5 nm in diameter). In the case of nevus of Ito, the granular component appeared to predominate in the shealth, which measured up to 3 micrometers in width and was estimated to cover about 97% of the total surface of dermal melanocytes. No ultrastructural changes were appreciable on the melanocyte shealth in nevus of Ito according to the difference in the age of patients (39 and 64 years), or in the lasting period of the dermal melanocytosis. By contrast, the melanocyte sheath in the case of Mongolian spot was much less developed than in nevus of Ito, predominantly filamentous in texture, and showed a marked decline with advance of the age of infants (from 2 mo to 6 yr). Many of the dermal melanocytes in Mongolian spot showed a partial disruption of their cytolemma, resulting in discharges of melanosomes free into the connective tissue matrix. Significance of these findings was discussed in relation to the differtial fate of melanocytes within the human dermis.