This study clarified the structures and seasonal changes of the annual Zostera marina population from April 2003 to November 2004, growing at Tategami-ura, Ago bay, Mie prefecture. Productive structures showed the graminous types excluding flowering period (May-June). Biomass and shoot density reached maximum values in May-June. At that time, over 80% of shoots were flowering shoots. And then, all shoots were disappeared in summer. From October-November, a lot of seedlings recruited and grew to flowering shoots by next spring. Maximum daily net production showed 4.50 g m^-2 day^-1 in June, which was similar level to that of the perennial Zostera marina population. However, yearly net production (305.0 g m^-2 year^-1) showed lower than that of the perennial type. Yearly maximum biomass was 152.6 g m^-2, and production/biomass (P / B) ratio was 2.0. It is necessary to more research in the production because the biomass and shoot density of the annual population verify drastically year to year.