Concentrations of mercury, cadmium, lead and copper are determined in the surrounding seawater and in seaweeds, Undarta pinnatifida and Sargassun fulvellum, from Suyeong Bay in Busan in the spring tide and neap tide from January to April 1974. The range and mean of the heavy metal concentrations in the surrounding seawater are as follows : mercury 0.00-0.39 ppb, 0. 16ppb; cadmium 0.00-0.46 ppb, 0.18 ppb, lead 0.00-0.94 ppb, 0.26 ppb : copper 0.00-0.86 ppb, 0.25 ppb respectively, and the concentrations varied slightly according to the tide. The mean values of concentration rate of Hg, Cd, Pb and Cu in air dry base were $0.42 imes10^3(0.13 imes10^3~1.0 imes10^3)$, $2.1 imes10^3(0.8 imes10^3~4.9 imes10^3)$, $8.9 imes10^3(3.1 imes10^3~19 imes10^3)$ and $15 imes10^3(6.0 imes10^3~28 imes10^3)$ in the Undaria pinnatifida, and $0.25 imes10^3($0.06 imes10^3~0.56 imes10^3)$, $1.0 imes10^3(0.61 imes10^3~1.7 imes10^3)$, $5.4 imes10^3(3.1 imes10^3~8.5 imes10^3)$ and $22.8 imes10^3(14.4 imes10^3~52.4 imes10^3)$ in the Sargassun fulvellum. The concentration rate of Hg, Cd and Pb of the Undaria pinnatifida was almost twice as much as that of the Sargassun fulvellum but the concentration rate of copper of the former was slightly smaller than of the latter.