In order to examine the relationship between crowding and growth rate in green hydra (Hydra viridis), we raised animals at various levels of fixed population densities; all new individuals in excess of the fixed densities were counted and removed every 4 days. A significant inverse relationship between population density and population growth rate exists. In addition, hydras were found to increase or decrease their growth rates in response to rapid changes of density after acclimation to fixed densities. The most dramatic effects were noted when the changes in degrees of crowding were greatest. Investigation of the effects of crowding in hydras on the total quantity of algal endosymbionts and their pigments revealed no significant changes in algal numbers or in carotenoid to chlorophyll-tf ratios at any level of crowding. Data on the presence of a water borne inhibitor of asexual reproduction supported the hypothesis that crowded culture medium contains a substance which depresses growth rates in green hydras
