On the community of motile animals inhabiting the seaweed Sargassum cymosum C. Agardh, at Ubatuba, SP, Brazil.Twenty-nine samples of marine animals, mainly invertebrates, inhabiting the seaweed Sargassum cymosum were collected in 1967 and 1968, at Ubatuba, Enseada do Flamengo (23° 30' S and 45°' 4' 20" W). They have been studied qualitatively and quantitatively. The main conclusions are: (1) Out of 19,193 animals, 50.2% were Mollusca, 18.8% Polychaeta, 18.7% Crustacea, 10.6% Echinodermata, and 1.7% various other groups. As most of the Foraminifera, Copepoda, Ostracoda, Nematoda, and Acarina were less than 0.2 mm, none were counted in the total or percentages. This is especially important for the percentage given here for Crustacea.(2) The distribution of frequencies of animals as a function of the hour of sampling shows that there is a strong diurnal migration of the fauna as a whole. Between 6 and 10 h there is a migration down from the seaweed to the rocky substrate, and between 18 and 22 h, the seaweed is again invaded by animals which stay through the night. (3) It is suggested that the animals respond to changes in the concentration of free O 2 and CO 2 in the seawater, factors which vary according to the photosynthetic activity of the plants. The descent out of the seaweeds by day, which in certain animals occurs against a positive phototaxy, is considered as a search for food on the surface of the rocks on which S. cymosum grows, and this movement is only possible during the day when seaweeds are producing oxygen by photosynthesis. S. cymosum by itself is not used as food, except in a few cases by young Lytechinus variegatus, and never significantly. The invasion of seaweeds by night is necessary for reaching water with more oxygen and less carbon dioxide brought to the upper surfaces of the plant layer by waves. (4) Despite considerable variability, periods of high seasonal densities could be noted for some of the dominant animal species represented in the samples. Most of the populations increased in the spring and continued high during the summer, then began to diminish in autumn to reach their lowest densities in early winter. (5) Mollusca were studied in more detail: Prosobranchia represented 92.8% of the individuals, Opisthobranchia 6.0% and Pelecypoda 1.2% (6) The dominant species was Tricolia affinis (C. B. Adams, 1850), a prosobranch representing 18.1% of the whole community. The diurnal migrations of this species were peculiar in showing late ascent onto the seaweeds in the evening. Perhaps T. affinis is more resistant than other organisms to an increase of CO 2 or a decrease of O 2 , and thus can stay on the rocks longer and compete better for food with the rest of the community. * Contribution no. 23 du Programme d