Chloroplasts from siphonaceous algae remain photosynthetically active in the digestive epithelial cells of the sacoglossan opistobranchs Elysia viridis and Tridachia crispata for up to 3 months. During this time, chlorophylls and glycolipids are not synthesized, but the synthesis of small molecular weight metabolites such as sugars, amino acids etc. can readily be demonstrated. Analysis of the capacity of symbiotic chloroplasts to synthesize ribulose diphosphate carboxylase and membrane proteins produced negative results in both instances, even though photosynthetic CO2 fixation could always be demonstrated. These findings show that symbiotic chloroplasts are biochemically stable entities, and that they neither grow nor divide while in the animals’cells. It it likely that chloroplasts depend on the plant cell nucleus and cytoplasmic protein synthesizing machinery for the synthesis of many of their structural and enzymatic components.