The development of the retinal projections to the roof of the midbrain was studied in Syrian hamsters after right superior colliculus (SC) lesions on the day of birth, using both autoradiographic and degeneration techniques. The dead tissue resulting from the heat lesion is not completely removed until the eighth day after birth. Normally the midline of the SC is defined by a pia-lined fissure separating the left and right colliculi, but in the animals with early unilateral lesions, the pia at the midline is damaged. When it regrows, together with vascular and other meningeal tissues, it forms a flat tissue bridge across the midline as early as two days after the lesion. When the axons from the left eye reach the right SC, they encounter the dead tissue and separate into two bundles. One bundle courses over the surface of the dead tissue and one grows underneath it. It is not until the third to fourth day that axons in the dorsal bundle cross the midline, via the tissue bridge, to terminate anomalously in the medial wall of the left SC. When the quantity of such recrossing axons is small, they overlap extensively with the optic tract fibers from the other (right) eye which normally have innervated the entire SC by day 3. However, it appears that as the density of the recrossing axons increases they displace the axons originating in the other eye from the medial wall of the left SC. Thus, eventually fibers from both eyes terminate in the left SC, occupying separate territories with little, if any, overlap. Axons in the ventral bundle begin to innervate the deep layers of the right SC on day 2. These axons were never observed to recross the midline. These results indicate that mechanical guidance and axonal segregation dependent on relative densities are two processes that govern the development of retinotectal projections after early SC lesions in hamsters.