The purpose of this experiment was to study the diameter of axons at the time of the initiation of myelin and the pattern of growth of axons in the optic nerve of the chick. Embryos between 15 and 20 days and chicks 3, 5, 22 and 60 days of age were studied on the electron microscopic level. Based on axon diameter a unimodal distribution of unmyelinated axons is present through day 20 of incubation with a mean of approximately 0.35 micrometer. This population is represented through 22 days of age but from day 3 on, a second distinct population of unmyelinated axons is present which has a mean diameter that is approximately twice that of the smaller unmyelinated axons. All axons do not increase simultaneously in diameter but once growth starts, the unmyelinated axons apparently double in diameter at a relatively rapid rate prior to myelination. On incubation day 17 less than 1% of the axons in the optic nerve is myelimated. The number of axons in this group and their diameter (mean approximately 1.2 micrometer) remain relatively constant through day 3 but from days 5 through 22, two distinct populations of myelinated axons are present. By day 60, three distinct distributions of myelinated axons are present with mean diameters of 0.51 micrometer, 1.76 micrometer, and 3.90 micrometer. These populations represent approximately 20%, 67%, and 13% respectively of the total fiber population. As age increases the diameter of some myelinated axons is as small as or smaller than the unmyelinated axons at an earlier period in development. This suggests that factors other than axon diameter might be involved in the start of myelination. It appears that the increase in axon diameter does not occur in a continuous manner but in a saltatory manner from one size to another.