From December 2004 to November 2006, we monitored monthly the growth and gonad development of the sea urchin Hemicentrotus pulcherrimus in relation to an algal standing crop in an Undaria pinnatifida and Saccharina japonica kelp bed in Matsushima Bay, Miyagi Prefecture, Japan. The algal standing crop increased markedly from January or February to a maximum in July. U. pinnatifida was predominant until May or July, then S. japonica predominated until August or September. The sea urchins grew significantly from January to April 2005. The gonad index (gonad wet weight × 100/body wet weight) was lowest (∼10) as a result of spawning from December to February, rose rapidly to more than 20 in April, and then remained high until November. The nutritive phagocytes in the lumen of the gonad were rapidly renewed and surrounded relict gametes for long periods from March to June or July. High gut content index values (gut content wet weight × 100/body wet weight) from January to May suggest that the test growth and marked rise in gonad index after spawning were caused by high food availability (U. pinnatifida). It is likely that subsequent sustained high gonad index values until maturation were ensured by the abundance of S. japonica.