1. Paramecium bursaria ingests a variety of particles, but this ingestion is not random. The paramecia select between different particles of approximately the same size when present in approximately equal concentrations. 2. The paramecia ingested six of the seven species of algae tested. Five of these six are maintained to some extent in the cytoplasm of P. bursaria. Maintenance of the alga in the cytoplasm seems independent of the nutritional type of the alga. 3. Of all the algae tested, only the naturally-occurring species of Chlorella and a free-living strain of Chlorella vulgaris have been established as symbionts, the criterion being the distribution of algae to all of the progeny of the paramecium. The adaptations of the algae to this niche are discussed. 4. The role of (a) selective feeding by the paramecia, and (b) the physiological condition of the alga in establishing this relationship are discussed. 5. The nature of the interaction between Paramecium bursaria and its endocellular symbiont is discussed. Apparently this relationship is unusual since it lacks specificity and permanence, two traits characteristic of most symbioses.