In order to assess the quantitative rôle of energy fixed by symbiotic zooxanthellae in the nutrition of the anemone Anemonia sulcata (Pennant), budgets were compared for a group kept in darkness for 84 days with a group which received a 12-h daily exposure to light at 140 μE·m −2·s −1. Both groups were starved. Hence the only nutritional source for the group kept in the light were the organic compounds resulting from the photosynthesis of the zooxanthellae. The group kept in darkness was assumed to be receiving no nutritional inputs. The animals kept in darkness lost weight as tissue was metabolized, and zooxanthellae were lost. The calculated rate of energy loss closely balanced the rate of respiration, indicating that tissue catabolism was being used to fuel energy metabolism. Under these conditions it is possible that the zooxanthellae could be parasitic on the host. The animals kept in the light lost weight, but the numbers of zooxanthellae increased. The loss in weight was less than in the group kept in darkness. Although the animal was undergoing atrophy the rate of energy fixation in photosynthesis was in excess of that required for energy metabolism. Approximately 40% of the photosynthetically fixed energy was unaccounted for and presumably was excreted. It appears that although energy from photosynthesis can be supplied in excess of that used in respiration, for growth to take place there has to be an input from heterotrophic nutrition.