The Opisthobranch Molluscs Elysia timida, E. translucens, Thuridilla hopei and Bosellia mimetica feed suctorially on the algae Acetabularia acetabulum, Udotea petiolata, Cladophora vagabunda and Halimeda tuna, respectively. The animals retain tunctional chloroplasts from their food. Symbiotic retention of functional algal chloroplasts in these Sacoglossan species is proved by electron micrography of the digestive gland, and the activity of the organelles by means of incubations with NaH14CO3 in the light and in the dark for varying periods of time. In Elysia timida. fixation rates were measured at different light intensities and between 10-35ºC. Carbon incorporation after a 40 days starvation period is registered. About 7-8% of the net 14-C photosynthetically fixed by functional chloroplasts was secreted in mucus by the molluscs. After 12 days, juveniles retained chloroplasts in the digestive gland. Chloroplasts associated with Thuridilla hopei lose their functional ability after 5 days. The results are discussed in the general frame of the meaning of this symbiosis between algae and molluscs.