Vertical distribution of the sea urchin, Strongylocentrotus nudus, and vegetation in Sakazuki, Tomari, Hokkaido, Japan, was observed monthly during from December 2002 to September 2003. Sampling points were established in the kelp forests, Laminaria religiosa, (St. A) and the Sargassum forests, Sargassum confiisum, (St. B), respectively. Individual number of the sea urchin and coverage of the L. religiosa or S. confusum were recorded, along a belt transects (65m long×1m wide, 0 to 2.9m depth in St. A, 0 to 2.3m depth in St. B). Dominant bottom quality in two stations was rock with large angulations. At deeper zone rather than kelp or Sargassum forest, where curstose coralline dominates, individual density of the sea urchin does not increase over year. Individual density of the sea urchin in kelp bed decreased during from December to next May. In Sargassum bed, the density during from April to September is lower than that of other months.
