To understand conscious vision, scientists must elucidate how the brain selects specific visual signals for awareness. When different monocular patterns are presented to the two eyes, they rival for conscious expression such that only one monocular image is perceived at a time. Controversy surrounds whether this binocular rivalry reflects neural competition among pattern representations or monocular channels. Here we show that rivalry arises from interocular competition, using functional magnetic resonance imaging of activity in a monocular region of primary visual cortex corresponding to the blind spot. This cortical region greatly prefers stimulation of the ipsilateral eye to that of the blind-spot eye. Subjects reported their dominant percept while viewing rivalrous orthogonal gratings in the visual location corresponding to the blind spot and its surround. As predicted by interocular rivalry, the monocular blind-spot representation was activated when the ipsilateral grating became perceptually dominant and suppressed when the blind-spot grating became dominant. These responses were as large as those observed during actual alternations between the gratings, indicating that rivalry may be fully resolved in monocular visual cortex. Our findings provide the first physiological evidence, to our knowledge, that interocular competition mediates binocular rivalry, and indicate that V1 may be important in the selection and expression of conscious visual information.