An analysis was made of the drift of Ceratoneis arcus, in the river Kaltisjokk (Swedish Lapland, 66[degree]42[image]N.) throughout the annual cycle in 1965-66. The annual drift has 2 maxima, one in June-July and a weaker one in Oct. -Nov. Minima occur in Aug. and from Jan. until April. After the formation of auxospores which can be established by the size and form of Ceratoneis cells, drift increases again. The diurnal periodicity of Ceratoneis drift follows the pattern of day -active organisms. Seasonal adaptations in activity curves to the length of day are not apparent. In the middle of Dec. drift of Ceratoneis is desynchronized. The measurement of Ceratoneis drift at 3 points on the river permits a discussion of the distance covered by the drift algae.