We gauged the effect of the sea urchins Hemicentrotus pulcherrimus and Anthocidaris crassispina on an Eisenia bicyclis kelp bed on the Kii Peninsula in southwestern Japan. We monitored sea urchin density, size, and vertical algal distribution every 2 months from September 2000 to March 2002 and in September 2002 in a 40-m² plot that stretched from the intertidal (reef) zone to the subtidal zone at depth of up to 2 m. The sea urchin densities were more than 40 individuals/m² (H. pulcherrimus) and more than 5 individuals/m² (A. crassispina) during September 2000 to May 2001. These densities decreased abruptly to less than 20 and less than 1 individual/m² in October 2001, respectively, coincident with low salinities of less than 23 practical salinity unit (PSU) in late June because of river water inflow. Until May 2001, adult E. bicyclis grew only on the intertidal reef, and articulated coralline turf was predominant in the subtidal zone. From January 2002, the kelp bed expanded slightly to the upper subtidal zone, but most young kelp growing at a depth of more than 1.5 m disappeared, despite the low sea urchin density. It is likely that abiotic factors resulting from the river water inflow inhibited the growth of E. bicyclis.