To understand the mechanism by which dissolved inorganic nutrients cause Chlorella to overgrow Hydra digestive cells, the symbiosis between hydra (Hydra viridissima) and algae (Chlorella sp.) was simulated with a novel computer model. The model faithfully simulates the attainment of equilibrium populations of algae and hydra digestive cells. as well as overgrowth by algae. The model is used to discern the parameters that lead to instability of the symbiosis. Additional simulations suggest that several mechanisms acting together are required to produce overgrowth. These include the release of algae from density-dependent growth, entrance of an increased proportion of algae into division, division of an increase proportion of these into eight autospores. and a decrease in the host cell growth rate.