The organization of retinofugal fibres in the developing and adult mouse has been studied with transmission electron microscopy, autoradiography and the Bodian silver method. It has previously been shown that all retinal ganglion cell axons are in glial-wrapped bundles in the developing and adult optic nerve, but are not in similar bundles close to the chiasm. In the embryonic mouse this region shows a transition in glial morphology from an interfascicular to a radial type and here retinofugal fibres begin to form a new order related to their age. Growth cones become concentrated at the pial surface of the juxtachiasmatic nerve and older fibres are restricted to deeper regions. This same age-related order is also evident in the optic tract. However, the age-related order is lost within the chiasm, where growth cones, young and old fibres are again mingled in distinct bundles as they cross the mid-line. This study is particularly concerned with the structure of the mid-line bundles. These fibre bundles cross each other at right angles, and are recognizable in fetal and adult mice. In the adult, monocular injections of H3 proline followed by autoradiographic study show that the individual mid-line bundles are monocular and that they fuse again, losing the fascicular structure as they leave the chiasm and enter the tract. In the fetus and in the adult, the bundles generally lack a complete glial wrapping so that growth cones can lie in intimate contact with two crossing bundles, one coming from the left eye, the other from the right. The interesting question about the mechanisms that keep growth cones from entering the wrong bundles when they are in this position remains to be addressed.