On the basis of an electron microscopic examination of the optic nerve of the North American opossum it was estimated that the nerve has approximately 100,000 axons, of which 98% are myelinated. The myelinated axons ranged from 0.3 to 6.7 micrometers in diamter (mean 1.6 micrometers), while unmyelinated axons were 0.2 to 1.6 micrometers in diameter (mean 0.6 micrometers). Axoplasm and axon (axoplasm plus myelin) diameter spectrums were unimodal and positively skewed. The mean of the ratio of axoplasm to axon diameter was 0.69. However, this ratio varied widely across axons and was nonlinearly distributed, decreasing with axon diameter. An inverse relationship between axon density and a high proportion of large-caliber axons was located dorsally in a cross section obtained near the eye. However, in sections obtained near the optic chiasm, regions having the highest proportion of large-diameter axons were in the ventral periphery of the nerve. It is suggested that within the 40-mm length of the nerve, there may be a change from a retinotopic organization of axons according to their diameter and central targets.