Myelinogenesis in the optic nerve of the mouse begins by the end of the first week of postnatal life and proceeds well into adulthood. The optic nerve from different postnatal age groups of (C57BL x CBA) F1 hybrid mice was analysed ultrastructurally and morphometrically. Features examined were the myelinated and unmyelinated nerve fibres, mean myelinated nerve fibre diameter, myelinated nerve fibre diameter spectrum, number of myelin sheath lamellae in relation to fibre diameter and age. Results obtained showed that the optic nerve of the mouse is populated entirely by unmyelinated nerve fibres at birth. Myelinogenesis begins at about the fifth postnatal day (pnd), with clearer evidence by the 6th pnd. It starts selectively with the largest fibres and then progresses to involve other fibres of decreasing diameter. After the onset, myelinogenesis progresses with increasing rapidity, becoming most intense from the second week up to the fifth week, so that by the end of the fifth week about 73% of fibres have become myelinated. From then onward, the process proceeds at a progressively diminishing rate until, by the 16th week, virtually all the remaining 27% fibres are myelinated. At onset, the myelin sheath is loosely wrapped around the axons, but with increasing age the myelin lamellae become more compact around the axons. The population of unmyelinated fibres continued to diminish so that at the completion of the process (16th week), they were only very rarely encountered. Myelinogenesis in the optic nerve of the mouse appears to follow a similar course to that seen in other central tracts having a similar fibre diameter spectrum.