Similarities between optokinetic nystagmus (O.K.N.) and flicker-induced nystagmus (F.I.N.) include: response buildup and occurrence during the entire period of stimulation; absence of habituation; after-nystagmus in the same direction; occasional after-after-nystagmus to the opposite side; frequency spectrum, peak frequency and existence of lower and upper frequency thresholds. Phenomena are different in that for O.K.N. the stimulus is in motion; binocular and monocular stimulation are effective; direction is determined by direction of motion; unidirectionality is absent; there is minimal influence of background illumination, posture, labyrinthine receptors, and section of crossed optic fibers. For F.I.N. the stimulus is stationary; the response is elicited only by monocular stimulation; the direction is determined by the stimulated eye; there is exclusive unidirectionality; it is abolished by background illumination, optic chiasm section and bilateral labyrinthectomy; it is strongly influenced by posture. This comparison suggests that O.K.N. and F.I.N. are separate responses and that the effective stimulus for F.I.N. may activate selectively the crossed optic fibers.