Abstract: The immunostimulated blue algae is the transgenic Synechococcus sp. PCC7942 in which human thymosin-alpll (Tα1) gene had been expressed effectively. It has been confirmed that the immunocompetence in mice can be improved by oral administration of immunostimulated Synechococcus sp. PCC7942. The immunostimulated Synechococcus sp. was fed to Anguilla japonica as feed additive in this experiment. And the black fry was infected with epidemic pathogenic Vibrio sp. which had been isolated from the diseased A. japonica in the laboratory. Ta, contents in the measured tissues and organs of the black fry increased after fed with immunostimulated Synechococcus sp. The immunocompetence in A. japonica could be improved by oral administration of immunostimulated Synechococcus sp.. Resistances to bacterial pathogens such as Vibrio sp. and to environmental stress could be increased by fed with this immunostimulated blue algae, and the effects of the immunostimulated blue algae facilitated on growth of the black fry have been also observed in the experiment. The immunostimulated blue algae was the feasible aquatic feed additive and its dosages was about 0.5 to 1.0 gram per kilogram formulated feed.