The endosymbiotic unit green Paramecium shows a strong specificity of its partners. The aposymbiotic Paramecium bursaria forms a stable symbiotic unit only with a special strain of Chlorella sp. Algae suitable for symbiosis formation are enclosed in individual perialgal vacuoles whereas unsuitable algae are sequestered into food vacuoles. It is probable that algae are recognized by the ciliate because of specific surface structures rather than by their physiological properties. Experiments with synchronized algae demonstrate that autospores are taken up into perialgal vacuoles to a higher degree than mother cells, which have a different surface structure as shown by immunological techniques. Symbiotic algae treated with cellulase and pectinase or having been coated with specific antibodies or with lectins (concanavalin A or Ricinus communis agglutinin) are usually not recognized as suitable and are mostly sequestered into food vacuoles although they show the same physiological properties as untreated algae. These results indicate the participation of carbohydrate structures at the recognition sites of symbiotic chlorellae in Paramecium bursaria which interact during infection with special receptor molecules in the membrane of the ingestion vacuole of the ciliate.