The organization of retinotectal fibers in the optic chiasm was investigated using horseradish peroxidase (HRP) histochemistry and electrophysiological recording. HRP injection into a small region of the tectum led to retrograde staining of labeled fibers in a circumscribed region of the chiasm and staining of labeled ganglion cells in the contralateral retina. In each instance labeled tissue was spread over a greater proportion of the area of a chiasm section than over the flattened retina. Fibers originating in central (older) retina are located in dorsal chiasm. Fibers originating in peripheral (younger) retina are located in ventral chiasm. Viewed with the electron microscope, labeled unmyelinated fibers are admixed with labeled myelinated fibers. Neuronal activity was monitored with an extracellular microelectrode from points in dorsoventral tracks in the chiasm. Multiple units were recorded at each chiasm location. Using visual stimuli, the receptive fields of the units were mapped. The fields were distributed along an arc across the visual field. At ventral chiasm recording sites the arc was in the peripheral part of the visual field. In succeeding dorsal sites the arcs were concentrically arranged so that the more dorsal the chiasm recording site, the more central was the arc in the visual field. Thus, in the optic chiasm, retinal fibers appear to be organized chronotopically but not retinotopically. Fibers of the same age but from different locations in the retina are mixed together.