In the developing mammalian visual system, retinal fibers grow through the optic chiasm, where one population crosses to the opposite side of the brain and the other does not. Evidence from labeling growing retinal axons with the carbocyanine dye Dil in mouse embryos indicates that the two subpopulations diverge at a zone along the midline of the optic chiasm. At the border of this zone, crossed fibers grow directly across, whereas uncrossed fibers turn back, developing highly complex terminations with bifurcating and wide-ranging growth cones. When one eye is removed at early stages, uncrossed fibers from the remaining eye stall at the chiasm midline. These results suggest that crossed and uncrossed retinal fibers respond differently to cues along the midline of the chiasm and that the uncrossed fibers from one eye grow along crossed fibers from the other eye, both guidance mechanisms contributing to the establishment of the bilateral pattern of visual projections in mammalian brain.