The total number of myelinated and unmyelinated nerve fibers was counted in a montage of electron micrographs prepared to cover the entire profile of the mouse optic nerve in transverse section. Many astrocytic processes form a barrier structure all around the optic nerve. As a whole, the mouse optic nerve consists of one fascicle where the nerve fibers, the glia cells and their processes, and the capillaries are fitted closely together to fill substantially all of the available space. No tissue space accompanied with connective tissue septa penetrates into the fascicle, except for those situated around the arteriole and venule. The total number of 64,746 myelinated nerve fibers in the optic nerve in one side was counted. This corresponds to about 98.8% of the total number of the optic nerve fibers. The total number of 807 unmyelinated nerve fibers was also counted. This corresponds to about 1.2% of the total nerve fibers. The external diameter of every myelinated fiber, including the myelin sheath, was measured on the montage micrograph and its frequency distribution was examined. The diameter of the myelinated fibers ranges from 0.3 to 4.2 mu and its mean value is 0.96 mu. The frequency distribution of fiber diameter is unimodal with a peak at 0.7 to 0.9 mu. A regional heterogeneity in fiber size is found in the optic nerve. The nerve fibers in the peripheral area of the nerve are relatively small and uniform in diameter, whereas those in the central area show a fairly wide range in diameter.