1. The tropical coral reef-dwelling coelenterateZoanthus sociatus (Ellis) lives in mutualistic symbiosis with dinoflagellates of the genusGymnodinium. These algae are intracellular. 2. Analysis of the photosynthetic contribution of these endosymbionts shows a direct transfer of photosynthate from algae to animal and utilization of such substances by the animal. Such transfer does not involve destruction of the algae. In vitro studies of the photosynthetic products of the algae show that they synthesize a wide range of metabolites, but selectively release only a few including glycerol, glucose and alanine. These data indicate that such organisms possess an autotrophic mode of nutrition. 3.Z. sociatus shows a well defined feeding behaviour when offered homogenates ofEchinometra eggs. A similar behaviour may be elicited with reduced glutathione. Proline and glycine produced “mouth opening” responses but not the complete feeding response. Alanine, glutamic acid and aspartic acid gave no response. 4. The ultrastructure of the mesenterial filaments shows that the tissue is well differentiated and possesses nematocysts (holotrichous isorhyzas). Several distinct cell types including mucus secreting and “zymogen” cells have been recognized. These cells probably play an important role in extracellular digestion. 5. The rate of digestion of exogenously supplied proteins byZ. sociatus compares favourably with that of sea anemones and corals. Digestion is both extracellular and intracellular, the latter process taking place after particle phagocytosis in the “digestive-excretory” cells of the mesenteries proximal to the filament. 6.Z. sociatus can absorb dissolved amino acids and sugars from very low concentrations (10−6–10−7M), and may incorporate such metabolites particularly into reproductive tissue. 7. The pycnotic zooxanthellae found in the cells of the “digestive-excretory” zone of the mesenteries are probably derived from other areas of the animal gastroderm. These algae appear to undergo senesence in this tissue, but are not digested by the animals. This is very likely a normal phenomenon, in which case the “digestive-excretory” zone of the filament could be regarded as a “grave yard” for old defunct members of the algal population. 8. The polytrophic habit of reef-dwelling invertebrates with photosynthetic endosymbionts is viewed as an important parameter of coral reef nutrition, lending a great deal of nutritional versatility to animals. Such plasticity is probably a reflection of microinstability within an overall stable ecosystem.