The representation of the visual field in the upper layers of the opossum's superior colliculus was studied by recording the response of multi-units to visual stimulation. Overall, the visual topography is similar to that of other non-primate mammals. Along the horizontal meridian and magnification factor decreases assymetrically about the vertical meridian, falling more abruptly in the region of the representation of the ipsilateral hemifield. The colliculus may be divided into three parts on the basis of the projections from the retina: a rostral region that does not receive any ipsilateral retinal afferents, a region that receives binocular retinal projections and a monocular region that receives only contralateral retinal input. Electrophysiological recording revealed that the rostral region contains a representation of 40 degrees of the ipsilateral field. The representation of the vertical meridian forms the border between the rostral region and the binocular region and the representation of the 40-45 degrees longitude meridian forms the border between the binocular and monocular regions.