Ecological traits of five Zostera marina populations in and around Hiroshima Bay were investigated in a luxuriant season of 2004. Study sites were located at the most inner (Ajina; St.1), center (O-kurokami-jima; St. 2) and near the mouth area (northern coast of Yahiro-jima; St. 3) of Hiroshima Bay, and at the inner area (Ihono-sho; St. 4) and the outside (Heigun-jima; St.5) of Yanai Bay which is the strait adjacent to Hiroshima Bay. The distribution depth (C.D.L.) was different among the populations. The lower depth of Zostera distribution was limited to 1-2 m at St.1 and 2 by algal-mat of Ulva spp. and other macroalgae, though it was 4-6 m at St. 3-5. Mean biomass was ranged in 120-180g DWm-2, but shoot size and density was different among the populations. Mean density ranged between 88-278 shoots m-2, and the populations at St.3 and 5 exhibited higher densities with smaller shoots especially in shallow stands of the two population. These stands also exhibited higher proportion of below-ground biomass to total biomass. On the contrary, populations in inner areas (St. 1 and 4) exhibited lower density with larger shoots. Physical conditions such as water motion induced by waves were different among the habitats which were shown in grain size compositions of the sediments, and this could affect the ecological traits of each Z. marina population.