The region of retinal disparity that is effective in maintaining binocular alignment of the eyes was investigated by measuring the horizontal fusional amplitudes for fixation targets consisting of a small central cross to which peripheral lines having various disparities were added. It was found that the addition of peripheral targets significantly facilitated binocular alignment, but only if the peripheral lines had less than about 0.5 degrees of disparity. This result indicates that the fusional mechanism responsible for binocular alignment is only narrowly tuned for retinal disparity. In light of current evidence that indicates that much larger disparities are capable of initiating vergence eye movements, the results support suggestions that fusional vergence consists of functionally dichotomous vergence initiating (coarse) and sustaining (fine) channels.