This research on the binocular fusion of phenomenal yellow, given red-filtered flashes of light in one eye and green-filtered flashes of light in the other, directly compared the effects of wider-bandwidth and narrow-bandwidth filtering. 20 male college students with normal stereopsis, Mage = 19.3 yr., SD = 2.2, were tested for the binocular perception of flashing yellow sensations given wider-bandwidth versus narrow-bandwidth filtering of light flashes which, monocularly, were experienced as red sensations in one eye and as green sensations in the other. When 3 wide-bandwidth tests for binocular yellow fusion were alternated with 3 narrow-bandwidth tests, simultaneous flashes of red-filtered light in one eye and green-filtered light in the other were binocularly perceived as yellowish on 25% of the wide-bandwidth tests (SD = 40%)--as yellow on 8% of the tests, orange on 12% of the tests, yellow-green on 5% of the tests-and were binocularly perceived as yellowish on 0% of the narrow-bandwidth tests. When wider-bandwidth and narrow-bandwidth testing were separated spatially and conducted contemporaneously, the red-filtered flashes in one eye and green-filtered flashes in the other were binocularly experienced as yellowish sensations by 80% of all participants under wider-bandwidth conditions--as yellow by 55%, orange by 10%, yellow-green by 15%--and as yellowish sensations by 0% of the participants under narrow-bandwidth conditions.