The productivity and growth pattern of Sargassum confusum (Phaeophyta, Sargassaceae) was investigated in Ohori, east coast of Korea, from March 1983 to April 1984. S. confusum shows an obvious seasonal variation of the growth by passing through the germinative, vegetative, reproductive and decaying phases for about an year. The mean length of tagged individuals shows a positive correlation with the water temperature during the growing period. The growth is depressed at less than 1$0^{circ}C$ in February, and accelerated at above 15$^{circ}C$ in May. The rapid growth (1.29 cm/dry) occurs at 15-l8$^{circ}C$ during May-June and the maximum length is shown at 20-21$^{circ}C$ in early August. Daily net production estimated by the oxygen light and dark bottle method is 11.2 g C/$m^2$/ day in June. The annual value calculated by the integration of the daily net production, growth rate and biomass is 745 gC/$m^2$/yr. Net production estimated from the biomass change only is 287 gC/$m^2$/yr. But this must be an underestimated value, because the loss of considerable biomass by shedding is not included.