The interocular transfer of the motion aftereffect (MAE) was used to probe for the existence of binocularity in subjects with strabismus. The interocular transfer was measured with coarse and fine gratings subtending 2° and 8° of visual angle. Strabismic subjects were categorized by the extent of their binocular vision, particularly with respect to extrafoveal fusion and retinal correspondence. Subjects classified as having the monofixation syndrome consistently showed a substantial, though less than normal, amount of transfer with the larger 8° gratings. This is consistent with the peripheral binocular cooperation usually found in these subjects, and establishes interocular transfer of the MAE as a suitable test for binocularity in strabismus. The test is especially suitable in patients with suppression in which an existing binocularity may be masked by suppression on clinical testing. The tests were found to be applicable in children as young as eight years. MAE tansfer for large coarse gratings is believed to imply the existence of an early cortical binocularity, which provides the potential for sufficient binocular cooperation to permit stable ocular alignment.