Sea urchins, Strongylocentrotus nudus, were collected every month from a kelp bed and its neighboring coralline flat on the southwest coast of Hokkaido. The growth of the sea urchins from the two habitats with differing food conditions, rich (kelp bed) and poor (coralline flat), was compared. The age of the urchins was determined from the annual rings in the fifth genital plate. The average of the test diameters was greater at the kelp bed than those at the coralline flat for the ages under eight years (P<0.01 or P<0.05), but there was no statistical difference between the two habitats for ages over nine years. From the comparison of the ignition loss of the gut contents of the sea urchin in these two habitats, the food condition at the kelp bed was considered to be better than at the coralline flat. From these results, the relationship between the growth of the sea urchins and the food conditions of these two habitats were discussed.