In symbiotic associations between unicellular algae and aquatic invertebrates a relatively constant biomass ratio of algae to host is maintained. The mechanisms that maintain this ratio have not been adequately investigated. This study describes aspects of the mechanisms regulating numbers of algae in Hydra viridis digestive cells. Symbiotic digestive cells that acquire supernumerary algae by phagocytosis restore the number to normal levels by several mechanisms. Light and electron microscopy provide evidence showing that excess algae are expelled intact or digested within the host cells. This finding is significant since, under normal circumstances, symbiotic algae are not expelled and avoid digestion by inhibiting phagosome--lysosome fusion. The role of host cell mitosis in the regulation of the number of algae per cell is also investigated. Quantitative data are presented which show that mitosis alone cannot account for the regulation of supernumerary algae. The hypothesis that the supply of inorganic nutrients may influence the regulation of the number of algae per digestive cell is also tested in this study. Enrichment of the maintenance medium with a combination of nutrients enhances growth of the symbiotic algae, the digestive cells become packed with algae, and eventually the host is overgrown and killed by the algae. Therefore regulatory mechanisms controlling the algal population when exogenous inorganic nutrients are limited, are no longer effective when nutrients are available.