The zebrafish optic tectum is anatomically similar to those of goldfish and other teleosts, both in its laminar structure and the morphology of intrinsic neurons as studied with Golgi stains. We have applied standard electrophysiological techniques to study the visual properties of tectal cells, utilizing a computer system for stimulus control and data recording. All tectal cells have very large receptive fields, averaging 25-39 degrees in linear dimensions. Retinal receptive fields are smaller, averaging 7-13 degrees. In many cases the receptive fields of tectal cells, but never of retinal cells, consist of two parts (main field and accessory field) separated by tens of degrees. The two parts are differentially adapted by background illumination, accessory fields becoming unresponsive under lit conditions while main fields do not. This may reflect separate retinal input channels. Four types of tectal cells are described, which differ in their spontaneous activity in the dark and response to stationary spots. Type I are not spontaneously active in the dark, but respond phasically at response to ON and OFF. Type T are tonically active and give more prolonged phasic responses to ON and OFF. They may also have pure-inhibitory receptive fields in which spot ON suppresses the spontaneous firing with no phasic excitation. Type S are also silent in the dark, but give sustained firing as long as a spot is ON in the receptive field. Cells of type B fire spontaneously in bursts; the burst rate may be raised or lowered by stationary spots, but there is no phasic response. Each of the four physiological types is found to occur among the cells of the periventricular layer, all of which share a stereotyped overall morphology. Tectal cells do not exhibit spatially separated ON and OFF areas or orientation specificity.