The ciliateClimacostomum virens forms an endosymbiotic association with coccoid chlorophycean algae which can be isolated and grown as sterile mass cultures with an inorganic medium. According to both morphological and physiological properties, the algae probably belong to the genusChlorella and have some features in common with symbiotic chlorellae isolated from the ciliatesParamecium bursaria andStentor polymorphus. These and other endosymbiotic chlorellae studied so far excrete maltose or glucose, but algae fromClimacostomum virens show a different excretion pattern by releasing glucose, fructose and xylose. Possible biosynthetic pathways are discussed. Algae inClimacostomum virens are either located individually in special perialgal vacuoles where they are probably protected against attack by host lytic enzymes or to a lesser extent in food vacuoles in different states of digestion. The endosymbiotic character of theClimacostomum virens-Chlorella sp.-association is discussed.