Tokyo Bay is one of the most eutrophicated bays in Japan, and mass occurrences of scyphozoan jellyfish Aurelia aurita have often been observed with progress of eutrophication. In the coastal environment, the abundance of substrates suitable for A. aurita polyps are obviously increasing by reclamation. However, most suitable substrates for settling, except in the bottom-layer in the innermost part of Tokyo Bay, are occupied by other sessile organisms such as mussels, barnacles or ascidians. The bottom layer is characterized by low DO concentrations (Ϲ2.0 ml O 2 L Ϫ1) that turn to hypoxia during summer, resulting in low recruitment and low growth of other sessile organisms. To elucidate the mechanism by which mass occurrence of jellyfish occurs, adaptative behavior to hypoxia in planula larvae and the polyp stage was experimentally studied. In the present study, planula settlement was promoted by a decrease in DO concentrations, and apparent growth of polyps and production of daughter polyps by asexual budding were also observed, even in the polyps cultured in hypoxic waters. Moreover A. aurita polyps cultured under hypoxic conditions began strobilation and ephyrae were liberated after the DO concentration levels were restored. It is suggested that the occurrence of hypoxic waters is rather favorable for the survival, growth and asexual reproduction of A. aurita polyps, and it can directly contribute to the consequent production of A. aurita ephyrae in the next spring within Tokyo Bay.