A model is proposed for the spatial and temporal characteristics of X- and Y-type responses of ganglion cells in the primate retina. The model is related to a theory of directional selectivity proposed by Marr & Ullman (1981). The X- and Y-type responses predicted by the model to a variety of stimuli are examined and compared with electrophysiological recordings. A number of implications and predictions are discussed.Updated October 1981