The present paper extends the work published in an earlier account by Harris & Wolfe (1955). Vertical migration of Daphnia magna Straus was there shown to be controlled by two independent mechanisms, a photokinetic one responsible for a 'dawn rise' and a phototactic one which was linked to the orientation response. It is shown that Daphnia whose eyes have been removed exhibit the photokinetic response, including an inhibition of movement at high light intensities, but no phototactic response. Consequently such animals can carry out a vertical migration determined solely by light intensity. This mechanism accounts for vertical migration in animals without a complex visual apparatus; the photosensitive system involved resides in some quite different and perhaps more widely distributed general sensitivity to light.