Both the visual acuity of sustained-X cells in the area centralis of the retina and the binocular properties of visual cortical cells were studied in control cats, and in cats reared with divergent or convergent squint in one eye or with uniocular or binocular atropinization. The sustained-X cell acuity was normal in control cats with squint but with alternating fixation. The cellular acuity was reduced significantly in the squinting eye of the cats which lost the ability to fix with the deviating eye and in all penalized eyes of the cats regardless of whether one or both eyes had been atropinized. However, regardless of whether X cell amblyopia was present or not, all squinting cats and cats with uniocular penalization showed a significant reduction in the number of binocularly driven cortical cells. Nevertheless, the amblyopia of X cells in the retina was correlated with a loss of proportion of cortical cells driven by the amblyopic eye compared with that driven by the non-amblyopic eye. Thus amblyopia and loss of cortical binocularity (not stereo acuity) appear to have different physiological bases.