The early myelination of the dorsal funiculus at the level of the 4th cervical spinal cord was ultrastructurally studied in the one-day-old mouse. It was found that the fibers were mainly unmyelinated. However, some early myelinated fibers were scattered among unmyelinated fibers. In the initial stage of the myelination, the axon was partially contracted by a piece of cytoplasmic process of the oligodendroglial cell. The two lips of the oligodendroglial process then extended and converged, enwrapping the axon completely and forming the first contact point. With the anchorage of that contact point, the two lips of the process became elongated and enfolded by each other, and produced the internal and external tongues of the future myelin sheath. More contact points were formed at a regular interval by the regional fusion of the two external surface layers of the opposed cytoplasmic membranes of adjacent tongue processes. With the advanced bidirectional spiralization of the two tongue processes, many contact points were found between the adjacent lamellae of the concentrically arranged oligodendroglial process; simultaneously, the cleft between the neighboring contact points disappeared and formed the initial sites of the intraperiod line. During the early myelination, one single axon ensheathed concentrically by two different oligodendroglial processes as well as several axons enwrapped by a continuous spiral myelin sheath of one oligodendroglial cell were frequently observed. The cross-sectional areas of unmyelinated axons varied from 0.01 to 0.2 micron 2, with a median of 0.07 micron 2; whereas, that of promyelinated axons ranged from 0.09 to 1.4 micron 2, with a median at 0.61 micron 2. These data support the suggestion that the axon calibre is a critical factor for the initiation of central myelination.