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The classification of cat retinal ganglion cells as X or Y on the basis of linearity or nonlinearity of spatial summation has been confirmed and extended. Recordings were taken from optic tract fibres of anaesthetized, paralysed cats. 2. When an alternating phase sine wave grating was used as a stimulus, X cells had null positions and Y cells responded at all positions of the grating. 3. These results did not depend on the temporal wave form or the temporal frequency of pattern alternation over a wide range. 4. At high spatial frequencies for the particular cell, a Y cell gave abig 'on‐off' response, or frequency doubling, at all positions of the grating, while an X cell did not. 5. The use of contrast sensitivity versus spatial phase also served to differentiate the two cell types. With an alternating sine grating stimulus X cells had a sinusoidal dependence on spatial phase, while each Y cell's sensitivity depended in a complicated …