Heavy metal ions ($Pb^{2+}$, $Cd^{2+}$, $Mn^{2+}$, $Cu^{2+}$, and $Cr_2O{_7}^{2-}$) were biosorbed by brown seaweeds (Hizikia fusiformis, Laminaria japonica, and Undaria pinnatifida) collected from the southern coast of South Korea. The biosorption of heavy metal ions was pH-dependent showing a minimum absorption at pH 2 and a maximum biosorption at pH 4 ($Pb^{2+}$, $Cd^{2+}$, $Mn^{2+}$, and $Cr_2O{_7}^{2-}$) or pH 6 ($Cu^{2+}$). Biosorption increased most noticeably for pH changes from 2 to 3. In the latter pH range, biosorption increased, because a higher pH decreased the electrostatic repulsion between metal ions and functional groups on the seaweed. In the pH range of 2 ~ 4, biosorption of negatively-charged chromium species ($Cr_2O{_7}^{2-}$) followed the pattern of positively-charged metal ions ($Pb^{2+}$, $Cd^{2+}$, $Mn^{2+}$, and $Cu^{2+}$). This suggests that the most prevalent chromium species were positively-charged $Cr^{3+}$, reduced from $Cr^{6+}$ in $Cr_2O{_7}^{2-}$. Whereas positively-charged heavy metal ions ($Pb^{2+}$, $Cd^{2+}$, $Mn^{2+}$, and $Cu^{2+}$) reached a plateau after the maximum level, biosorption of chromium ions decreased noticeably between pH 5 and 8. Kinetic data showed that biosorption by brown seaweed occurred rapidly during the first 10 min, and most of the heavy metals were bound to the seaweed within 30 min. Equilibrium adsorption data for a lead ion could fit well in the Langmuir and Freundlich isotherm models with regression coefficients ($R^2$) between 0.93 and 0.98.