The distribution of retinotectal afferents has been studied by autoradiography in 4 adult cats. The findings suggest that crossed and uncrossed retinal fibers terminate in a striking cluster-and-sheet pattern that varies systematically with respect to the retinotopic map of the colliculus. Following unilateral eye injection, labelling was most pronounced in the contralateral colliculus but a suprising volume of label appeared on the ipsilateral side in all cases in the form of dense clumps of silver grains separated by sparsely labelled zones. The contralateral projection appeared densest in the most superficial of the 3 laminae of the stratum griseum superficiale; appreciable labelling was present also in the middle lamina at all survival times used (23-72 h). Near the area centralis representation labelling in both contralateral tiers weakened markedly and local gaps appeared densest in the more dorsal band. Elsewhere, labelling in this dorsal band was generally dense, though sharply interrupted at the optic disc representation and in a curious, elongated lateral zone at mid-collicular levels. In the caudal half of the binocular zone rarefications or 'holes', about 200 mum wide, appeared in the more ventral tier between more densely labelled zones of roughly similar width. On the ipsilateral side, labelling was sparse or absent at the rostral and caudal collicular poles, and was also weak in the region of the area centralis representation save for occasional very superficial grain-clusters. Farther caudally, however, prominent approx. 200 mum wide 'puffs' of label marked the middle lamina of the superficial gray layer. The puffs were most regular in shape in the caudal half of the ipsilateral zone and these were spaced at roughly 200 mum intervals. Puffs lateral to the horizontal meridian representation tended to lie more dorsal than those medial to this line and some of the most lateral puffs at mid-collicular levels invaded the upper lamina of the superficial gray layer. The optic disc representation was marked by a column of label extending through the upper and middle laminae. Similar experiments in cat fetuses suggest that these staggered--and possible even complementary--patterns of crossed and uncrossed retinotectal projection are innate: ipsilateral 'puffs' of labelling and contralateral 'holes' appear in the superior colliculus at least one week before term, as does the ipsilateral filling-in and contralateral gap at the optic disc representation. These observations suggest that in the cat, a vertical as well as horizontal organization may characterize the superficial layers of the superior colliculus. The additional finding of a similar, interrupted puff-like pattern of labelling in the stratum griseum medium following injections in the region of the substantia nigra makes it likely that a somewhat comparable cluster-and-sheet organization may exist also in the deep collicular layers.