Ecklonia cava Kjellman (Laminariales, Phaeophyta), a perennial seaweed, is one of the main components of the marine forests along the Pacific coasts of Japan. Its standing crop attains a maximum in summer and a minimum in winter. This is due primarily to seasonal variation of pinnate blade biomass. The present investigation was conducted on an E. cava community in Nabeta Bay, Shimoda (central Japan), with monthly samplings at 5 m depth, in order to quantify the growth process of this species. In the laboratory several biometrical parameters were measured. Although there were individual variations among the samples, it was possible to define several trends in the growth cycle of E. cava on the basis of the monthly averages. The number of young bladelets was 0-7; they increased from autumn to the beginning of winter, remaining constant for a short period until spring when they began to decrease, reaching a minimum in late summer. The longest bladelet appeared in spring and early summer, while the shortest one was found in early winter. The dry weight per unit area of bladelets increased with growth, and was low (6-9 mg cm^-2) during winter-spring and high (18-21 mg cm^-2) during the reproductive season. Based on the results, 4 periods can be distinguished in the annual growth cycle of the E. cava blade: 1) period of active formation of new primary blade and bladelets; 2) period of active growth of bladelets; 3) period of thickening of bladelets and development of sori, and 4) period of decay of old bladelets.