Nine of 12 tested strains of the zooxanthella Symbiodinium (= Gymnodinium) microadriaticum can initiate symbiotic associations with an aposymbiotic clone of the sea anemone Aiptasia tagetes. The growth of each strain during the first two months of association is generally related to the isoenzyme similarity coefficient of the strain relative to strain A, the strain originally isolated from Aiptasia tagetes. The growth of heterologous algae in Aiptasia is not enhanced by previous association with Aiptasia tagetes. An hypothesis on the nature of specificity, which involves a recognition mechanism between algae and potential host and competition between endosymbiont strains within hosts, is presented. 1. Strains of the symbiotic dinoflagellate Symbiodinium microadriaticum that were isolated and cultured axenically demonstrated differences in their respective capacity to infect and repopulate aposymbiotic Aiptasia tagetes. 2. Cultured algae from Aiptasia tagetes rapidly reinfected Aiptasia tagetes. Such homologous infections rapidly achieved high stable population densities similar to those found in freshly collected animals. 3. Algal strains isolated from other hosts infected Aiptasia tagetes less readily and produced lower stable population densities. Generally, those strains that were biochemically (with respect to isoenzyme patterns) similar to the algae from Aiptasia tagetes infected Aiptasia tagetes more readily than those that were dissimilar. Three strains did not infect Aiptasia tagetes. 4. An hypothesis is presented that postulates that a recognition process determines initial infectivity. Possible competition between algal strains for space in a host is visualized as a function of differences in interactions between hosts and algal strains, the net result of which is expressed as specificity in such associations.