Seasonal variations in vegetative growth and production of two seagrass species, Zostera asiatica and Z. marina, were investigated in Akkeshi Bay, northern Japan. Z. asiatica, a threatened species in Japan, was dominant, occurring from the intertidal zone to the deepest edge of the seagrass bed (5 m deep), whereas Z. marina was restricted to the shallower edge of the bed (<2 m). Above ground biomass and above ground net production per shoot were greater for Z. asiatica than for Z. marina. In contrast, shoot density was 3- to 4-fold higher for Z. marina. Biomass and production were minimum in winter to early spring (January to March), and maximum in summer (June to July) for both species. Annual production per unit area of Z. asiatica was larger than that of Z. marina (2033 and 1354 g DW m-2 y-1, respectively). Our findings reveal contrasting growth patterns for the two species: Z. asiatica allocates more resources to enlarging shoot size, whereas Z. marina allocates more to increasing shoot density by clonal propagation of rhizomes. Seagrass beds consisting of Z. asiatica contributed importantly to coastal ecosystems in Akkeshi Bay area because of high productivity.