The impact of the dominant grazers (Clithon spp.) of epiphytic algae on the biomass and percentage cover of a population of the intertidal seagrass Zostera japonica was investigated at Lai Chi Wo, northeast Hong Kong. The population density of Clithon spp. followed a seasonal pattern, varying between <1400 in summer to >2800 individuals m-2 in winter. High population densities of the gastropods were temporally correlated with high biomass of epiphytic algae on the surface of Z. japonica. An exclusion experiment using full, half and open fences to manipulate the density of Clithon spp. resulted in significant differences in the biomass of the epiphytes between the full fences and the other two treatments after 47 days. The percentage cover of Z. japonica also decreased significantly in the full fence treatment after 120 days, as a result of increased sloughing of leaves. These results suggest that epiphytic algae and their grazers play an important role in the growth and survival of Z. japonica, and such interactions should be considered in the conservation of the species.